This is a phase 3, multi-center, randomized, placebo-controlled, double-blind, parallel-group study with an equal randomization among the Hemay005 high dose, lower dose and placebo treatment groups. After subject randomization, each subject will enter an core-treatment Phase for 12 weeks following an extended-treatment phase for another 40 weeks and a follow up phase for 4weeks.

This is a multi-center, randomized, double-blind, placebo-parallel controlled phase III clinical study. The study consists of four phases, namely the screening period, the core treatment period, the extension period, and the drug discontinuation observation period.

Screening period: All subjects will undergo a screening period for up to 8 weeks prior to the baseline visit (V2, randomization day, Day 0).

Core treatment period: Patients with Behçet's disease (BD) meeting the eligibility criteria upon screening will be randomized in a 1:1: 1 ratio to the Hemay005 Tablets 45 mg BID test group, Hemay005 Tablets 60 mg BID test group, or the placebo group. They will first be given escalating doses for 7 days; subsequently starting from Day 7, they will be given Hemay005 Tablets 45 mg BID or 60 mg BID or the placebo BID continuously until Week 12.

Extension period: Considering benefits for subjects in the placebo group, and to observe the efficacy and safety of long-term treatment, all subjects will enter a 40-week extension period at the end of the core treatment period. Subjects enrolled in the test groups for the core treatment period will continue treatment at the dose for the core treatment period for 40 weeks during the extension period. Subjects enrolled in the placebo group for the core treatment period will be randomized in a 1:1 ratio during the extension period to either the Hemay005 Tablets 45 mg BID test group or Hemay005 Tablets 60 mg BID test group for treatment for 40 weeks. For the first week of extended treatment, subjects previously enrolled in the placebo group will need to undergo the same dose titration phase as for the core treatment period (Days 0-6), so that the same dosing schedule as for the two treatment groups would be achieved by the 7th day, in an effort to mitigate the intolerabilities such as gastrointestinal reactions, thus further protecting subjects' safety. If, during the dose titration phase of the extension period or during extended treatment, the subject cannot tolerate the prescribed dose, this will be handled at the investigator's discretion using the same method as for the core treatment period.

Drug discontinuation observation period: All subjects in the study (including those who prematurely discontinued treatment for any reason) will be observed for 4 weeks following the end of the last study dose.

Inclusion Criteria:

1. Understanding and voluntarily signing the Informed Consent Form (ICF) for this study;

2. Age 18-75 years (inclusive), male or female;

3. Diagnosed as BD based on the ICBD-2013;

4. At least 2 oral ulcers present at V1 (screening), and:

   1. at least 2 oral ulcers present at V2 (the day of randomization) when V2 occurs 14-56 days after V1; OR
   2. at least 3 oral ulcers present at V2 (the day of randomization) when V2 occurs 0-13 days after V1;

5. Applicability of systemic treatment for oral ulcers: Based on the severity of the disease and the involved area, the investigator determines that the patient's oral ulceration is not suitable for topical treatment or that the patient's oral ulceration cannot be effectively controlled by topical treatment, so that systemic treatment is to be used;

6. Throughout the study period from signing of ICF through 3 months after the last study dose, women of childbearing potential and male subjects who have not undergone vasoligation should use effective contraceptive measures, including vasoligation, abstinence, intrauterine device (IUD), hormones (oral, patches, rings, injections, implants) and barrier methods (diaphragms, cervical caps, sponges, condoms);

7. Being able to comply with the follow-up schedule and other protocol requirements.

Exclusion Criteria:

1. Active lesions associated with BD in major organs requiring immunosuppressive treatment, e.g., those in lungs (e.g., pulmonary aneurysm), blood vessels (e.g., thrombophlebitis, recurrent malignant aneurysms), gastrointestinal tract (e.g., gastrointestinal ulcers), and central nervous system (e.g., meningoencephalitis); Note: Patients with refractory BD who experienced gastrointestinal perforation, active bleeding, or obstruction, etc. within 3 months prior to randomization are to be excluded.

2. Any clinically significant heart disease (including but not limited to: unstable ischemic heart disease, NYHA III/IV left ventricular failure, or myocardial infarction) or clinically significant 12-lead ECG abnormalities detected during the 6 months prior to screening, which, at the investigator's discretion, may put the subject at safety risk or may interfere with the study assessments;

3. Use of the following immunomodulatory therapies:

   • Colchicine within 7 days prior to randomization;

   • Perazathioprine, mycophenolate, baritinib, or tofacitinib within 10 days prior to randomization;

   • Cyclosporine, methotrexate, cyclophosphamide, thalidomide, or dapsone within 4 weeks (28 days) prior to randomization;

   • Biologics within 5 half-lives prior to randomization, e.g.:

     • Etanercept within 4 weeks prior to randomization;
     • Infliximab or leflunomide within 8 weeks prior to randomization;
     • Adalimumab, golimumab, abatacept, or tolizumab within 10 weeks prior to randomization;
     • Secukinumab within 6 months prior to randomization;

4. Intraarticular or systemic corticosteroid treatment prior to randomization and within 5 pharmacokinetic/pharmacodynamic half-lives; Note: For subjects with eye symptoms, glucocorticoid eye drops are allowed throughout the trial (except for within 24 hours prior to a trial visit).

5. Chinese patent medicines with immunomodulatory effect within 2 weeks prior to randomization; any Chinese pate nt medicines or decoctions within 2 weeks prior to randomization that might affect efficacy evaluation, or containing sinomenine, total glucoside of paeony, or tripterygium wilfordii, etc.;

6. Laboratory tests:

   • Hemoglobin ≤85g/L;
   • White blood cell count <3.0×10^9/L or >14×10^9/L;
   • Platelets <100×10^9/L;
   • Serum creatinine >1.5 mg/dL (>132.6 μmol/L);
   • Total bilirubin of >2.0 mg/dL (>34.2 μmol/L);
   • Both aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≥1.5×ULN; Note: The above tests can be repeated at most once during the screening period. If the result within 2 weeks prior to randomization falls into the specified range, the subject is eligible for the study;

7. Use of potent inducers of cytochrome P450 enzymes (e.g., rifampicin, phenobarbital, carbamazepine, phenytoin sodium) within 4 weeks prior to randomization;

8. Other autoimmune diseases or chronic inflammatory diseases associated with immunity, e.g., rheumatic fever, rheumatoid arthritis, systemic lupus erythematosus, dermatomyositis, multiple sclerosis, Sjögren's syndrome, and inflammatory bowel disease;

9. Currently active infections or recurrent bacterial, fungal, viral, mycobacterial or other infectious diseases (including but not limited to tuberculosis, atypical mycobacteriosis, hepatitis B, hepatitis C, herpes zoster, histoplasmosis, and coccidiosis; however, onychomycosis is excluded), which, at the investigator's discretion, may put the subject at safety risk; Note: Subjects positive for hepatitis B surface antigen (HBsAg) or hepatitis C virus (HCV) antibody, or with a history of active mycobacterial infection of any species (including Mycobacterium tuberculosis) within 3 years prior to screening visit should be excluded. Screening is permitted if the subject has been cured for at least 3 years prior to randomization with documentation available for verification;

10. Clinically significant chest X-ray or CT abnormalities, which, at the investigator's discretion, may put the subject at safety risk; Note: If a chest X-ray or CT was performed within 3 months prior to V1, the examination may be omitted for V1;

11. History of transplantation or immunodeficiency;

12. Positive for human immunodeficiency virus (HIV) antibody or treponema pallidum antibody test;

13. Currently having a malignant tumor, or a history of any malignant tumor within 5 years prior to screening (except for treatment-experienced squamous cell carcinoma in situ of the skin, basal cell carcinoma or cervical carcinoma in situ with no evidence of relapse within the past 12 months);

14. Use of any clinical investigational product within 4 weeks prior to randomization or 5 pharmacokinetic/pharmacodynamic half-lives, whichever is longer; Note: Subjects who have participated in HM005BD2S01 study are not eligible to participate in this trial;

15. Known allergy to the study drug or any of its components or allergic constitution;

16. A history of alcohol or drug abuse or dependence, or psychiatric disorder;

17. Any conditions that may interfere with oral drug absorption, e.g., subtotal gastrectomy, clinically significant diabetic gastrointestinal disease, or certain types of bariatric surgery such as gastric bypass surgery, not including procedures that simply separate the stomach into separate chambers such as gastric banding surgery;

18. Prior use of apremilast;

19. Female subjects who are pregnant or breast feeding;

20. Concomitant serious, progressive, or uncontrolled diseases, with which participation in the study may, at the investigator's discretion, put the subject at potential risk or affect the interpretation of study results.