INCLUSION CRITERIA  Age 18-75 years. No residual tumor (R0) after D2 or greater lymphadenectomy through laparotomy. According to the definition of the 8th edition of the AJCC Cancer Staging Manual, patients with gastric adenocarcinoma confirmed by histopathology, pathological stage II (T4aN0M0) and stage III, including gastroesophageal junction adenocarcinoma (GEJ) patients. ECOG performance status 0-1. No metastasis or recurrence as radiologically confirmed. Patients must have adequate organ function as assessed in the laboratory tests. Patients must provide informed consent for this study, and sign the written informed consent form voluntarily before the initiation of the study, and are willing and able to comply with the scheduled visits, treatment plan, laboratory examinations and other study procedures in the study. Female patients of childbearing age must take a serum pregnancy test within 7 days before randomization with negative results, and agree to adopt reliable and effective contraceptive methods during the study.  4.2 EXCLUSION CRITERIA  Previous use of non-surgical therapy for gastric adenocarcinoma. Having liver, peritoneal or distant metastasis. Inability to take the drug orally. Having postoperative complications that are not relieved at the time of randomization. Uncontrolled pericardial effusion or pleural effusion which required invasive treatment, and grade II or above peritoneal effusion (diagnosed clinically) present at screening. Presence of contraindicated chemotherapeutic drugs in this study and failure to receive the adjuvant therapeutic regimen in any group specified in the protocol. Having received any surgery not for gastric adenocarcinoma requiring general anesthesia within 28 days prior to randomization. Having malignant tumors other than gastric adenocarcinoma within 5 years before randomization. Active autoimmune disorders requiring systemic treatment. Patients with immunodeficiency or receiving long-term systemic steroid therapy. Concurrent diverticulitis or symptomatic gastrointestinal ulcer disease. Patients who are receiving or requiring anticoagulant therapy. Patients with serious cardiovascular and cerebrovascular diseases. Diabetes mellitus that is not effectively controlled. Active infections requiring treatment. ≥Grade 2 peripheral neuropathy. Patients with active tuberculosis or having received anti-tuberculosis therapy within one year prior to randomization. Patients currently having interstitial lung disease or having a history of interstitial lung disease. Hepatitis B, known positive for hepatitis B surface antigen (HBsAg) and/or hepatitis B core antibody (HBcAb), and HBV DNA≥1000cps/ml; hepatitis C, positive HCV RNA or RNA ≥1000cps/ml. Human immunodeficiency virus (HIV) antibody positive. Vaccination of any live vaccine within 4 weeks before randomization. Previous allogeneic bone marrow transplantation or solid organ transplantation. Previous treatment targeting PD-1 receptor or its ligand PD-L1 or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) receptor; Previous history of serious allergy to monoclonal antibody or other biological preparations. Having participated in other interventional clinical studies within 28 weeks before randomization. Having clinically significant underlying medical disease that may affect administration of study drug or compliance to the protocol, as judged by investigators. Other patients who are considered by investigators as inappropriate for enrollment.