This study will evaluate the efficacy and safety of SIM0718 in adolescents (12- < 18 years) and adults (18-75 years) with moderate to severe AD.

This is a randomized, double-blind, placebo-controlled multicenter clinical study to evaluate the efficacy and safety of SIM0718 in adolescents (12- < 18 years) and adults (18-75 years) with moderate to severe AD. Subjects who meet entry criteria will be randomized to receive either SIM0718 or matching placebo.The study includes a screening period, a treatment period and a follow-up period.

Inclusion Criteria:

1. Age ≥ 12 and ≤ 75 years, male or female, and body weight ≥ 40 kg at the screening visit.
2. Diagnosis of atopic dermatitis at screening (according to the American Academy of Dermatology Concordance Criteria, 2014), and: 1) Adult diagnosed AD for ≥ 12 months and adolescent diagnosed AD for ≥ 6 months prior to screening; 2) Inadequate response or intolerance to topical medications, or is medically inappropriate for topical treatment judged by the investigator within 6 months prior to Screening; 3) At Screening and Baseline,IGA score ≥3; 4)At Screening and Baseline, EASI score ≥ 16; 5) At Screening and Baseline,total AD involvement ≥ 10% BSA; 6) Baseline peak pruritus NRS score ≥ 4.

Exclusion Criteria:

1. Not enough washing-out period for previous therapy.
2. History of any of the following:

1) History of significant immune reactions (eg, serum sickness, anaphylaxis, or delayed hypersensitivity) to any other biologic agent or any excipient of SIM0718; 2) Presence of other active skin comorbidities other than AD that may interfere with study assessments, such as scabies, skin lymphoma, or psoriasis; 3) Active keratoconjunctivitis and atopic keratoconjunctivitis at screening; or previous history of recurrent keratoconjunctivitis and atopic keratoconjunctivitis; 4) Patients with active tuberculosis (TB), latent TB, or a history of nontuberculous mycobacterial infection at screening; 5) HBsAg positive at screening; or HBcAb positive with HBV-DNA positive; or hepatitis C antibody positive with HCV RNA polymerase chain reaction positive; or HIV serology positive; 6) Systemic treatment with antibiotics, antivirals, antiparasitic agents, antiprotozoal agents, or antifungal agents for infections within 4 weeks prior to baseline, or superficial skin infections within 1 week prior to baseline that could interfere with study assessments (subjects could be re-screened after resolution of infection); 7) History of parasitic infection within 6 months prior to baseline; 8) According to the investigator 's judgment, known or suspected history of immunosuppression within 6 months prior to baseline, including but not limited to history of invasive opportunistic infections, such as aspergillosis, coccidioidomycosis, histoplasmosis, HIV, listeriosis, pneumocystis disease or tuberculosis; or the presence of abnormal frequent recurrent or persistent infections; 9) History of malignancy within 5 years prior to screening.

3. Planned major surgical procedures during the study.

4. History of alcohol or drug abuse within 2 years before screening.

5. Any other condition that, in the judgment of the investigator, would make participation in this study inappropriate.

6. Female subjects of childbearing potential, who experience any of the following

• Positive serum pregnancy test or positive urine pregnancy test before baseline
• Pregnant or breastfeeding women, or women planning to become pregnant or breastfeeding during the study who are unwilling to use at least one highly effective form of birth control throughout the study and for 90 days after the last dose of study drug.