Inclusion Criteria:  Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) score of 0 or 1 Patients with expected survival time more than 6 months Patients after standard D2/R0 resection Postoperative histologically confirmed adenocarcinoma of the stomach or GEJ Positive lymph nodes more than 7, stage pN3 Patients without distant metastasis (M0) or M1 with abdominal exfoliated cell detection positive (CY1P0) Patients' physical condition and visceral function allows following adjuvant therapy, including chemotherapy, chemoradiotherapy and PD-1 inhibitor therapy. Patients' blood routine and biochemical indicators should meet the following standard: Hb≥90g/L, ANC≥1.5*10^9/L, PLT≥100*10^9/L, ALT & AST≤2.5 U/L, TB ≤ 1.5 UNL, serum creatinine<1 UNL. Patients who are willing to obey regimens during the study. Written informed consent is acquired before random entry, and patients should know that he/she has the right to quit, and following treatment won't be affected. Patients are willing to provide samples of blood and tissue.  Exclusion Criteria:  Patients with gross peritoneal metastasis (CY1P0 excluded) or distant metastasis. Patients who has received any anti-tumor therapy before surgery. Patients who had received radiotherapy for abdominal organs including stomach, liver, kidney, etc. Patients who had active systematic autoimmune diseases which need systematic treatment within 2 years before first medication in the study, substitutive therapy (such as thyroxine, insulin, etc) excluded. Patients diagnosed with immunodeficiency, or was receiving systematic glucocorticoid treatment or other immunosuppressive therapy within 7 days before medication, physiological dose of glucocorticoid is allowed (≤10 mg/d prednison or equivalent medication) Patients who have known severe allergic reaction (≥level 3) to anti-PD-1 monoclonal antibody, 5-FU, Oxaliplatin or any auxiliary material. Patient diagnosed with other malignant tumor in the past 5 years, excluding radical basal cell carcinoma of the skin and/or radical resected carcinoma in situ. Patient with severe vital organ failure. Pregnant or lactation period Patient with known mental illness or drug abuse that may influence compliance. Patient with known HIV infection, or active tuberculosis. Untreated active hepatitis B Patient with active HCV infection Uncontrolled complications Other situations that might disturb study results and compliance.