This study is a multicenter, randomized, double-blind, placebo-controlled Phase III clinical trial aimed at evaluating the efficacy, safety, pharmacokinetics (PK), pharmacodynamics (PD), and immunogenicity of SHR-1819 injection in patients with Seasonal Allergic Rhinitis (SAR).

Inclusion Criteria:

1. Age 18 to 75 years (inclusive) at the time of signing the Informed Consent Form, of any sex;
2. Meet the diagnostic criteria for Seasonal Allergic Rhinitis (SAR) as defined in the "Chinese Guidelines for Diagnosis and Treatment of Allergic Rhinitis (2022, Revised Edition)", with or without allergic conjunctivitis;
3. The investigator assesses that the participant has had a poor response to intranasal corticosteroids and/or other medications for allergic rhinitis (antihistamines, leukotriene receptor antagonists, etc.) during the same pollen season in previous years;
4. Participants exhibit an immunoglobulin E-mediated hypersensitivity reaction to at least one seasonal pollen allergen;
5. Willing and able to complete the patient diary as required by the protocol during the study; the participant voluntarily signs the Informed Consent Form (ICF) before any study-related procedures begin, can communicate smoothly with the investigator, and understands and is willing to strictly comply with the requirements of this clinical study protocol to complete the study;
6. Female participants of childbearing potential and male participants whose partners are women of childbearing potential must agree to use the contraceptive measures specified in the protocol from the signing of the ICF until 3 months after the last dose.

Exclusion Criteria:

1. At screening or within 2 weeks prior to the screening visit, the participant has other active rhinitis besides SAR;
2. At screening or within 2 weeks prior to the screening visit, there may be other nasal comorbidities or concurrent diseases/conditions that could affect the efficacy assessment, acute/chronic sinusitis, or upper respiratory tract infection;
3. Has glaucoma, cataract, ocular herpes simplex, other ocular infections such as infectious conjunctivitis, acute conjunctivitis, or acute keratitis at screening;
4. History of Vernal Keratoconjunctivitis (VKC) and/or Atopic Keratoconjunctivitis (AKC) within 6 months prior to the screening visit;
5. Use of Monoamine Oxidase Inhibitors within 2 weeks prior to the screening visit;
6. Initiation of immunotherapy within 4 weeks prior to the screening visit or planned receipt of immunotherapy during the study period;
7. Receipt of an investigational drug or medical device treatment within 8 weeks or 5 half-lives (if the half-life is known) prior to the screening visit, whichever is longer. Participants who participated in a clinical study but only signed the Informed Consent Form (ICF), which can demonstrate screening failure and no drug administration, may be included;
8. Vaccination with or exposure to other live or live attenuated vaccines within 3 months prior to the screening visit, or participation in a vaccine clinical study;
9. Underwent major surgery within 3 months prior to the screening visit, or plan to undergo major surgery during the study period;
10. Diagnosis of active tuberculosis (TB) within 6 months prior to the screening visit;
11. Malignancy or history of malignancy;
12. Pregnant or breastfeeding women;
13. The investigator judges that there are conditions affecting the safety and efficacy evaluation of the study drug, as well as any other conditions that may lead to the participant's non-compliance with the study procedures and diary completion.