Tislelizumab Plus FOLFOX Versus POF in the Treatment of Locally Advanced: a Multicenter, Open-label, Randomized Phase III Studyunresectable or Metastatic Gastric/Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria: * Patients with advanced unresectable, histologically confirmed adenocarcinoma of the gastric or gastroesophageal junction. * 18-70yeas. * ECOG PS 0-1. * No previous chemotherapy (perioperative chemotherapy, six months after fluorouracil alone or one year after oxaliplatin or a combination of taxoid and fluorouracil can be included), radiotherapy or immunotherapy. * With normal marrow, liver and renal function: a hemoglobin (HGB) of ≥100g/L (without blood transfusion during 14 days); a leucopenia count of ≥4.0×109/L; a platelet count of ≥100×109/L; a total bilirubin (TBil) of ≤1.5 upper normal limitation (UNL); a creatinine (Cr) of ≤ 1.5 UNL; a creatinine clearance rate ≥ 50ml/min (Cockcroft-Gault); a alanine aminotransferase (ALAT) and aspartate aminotransferase (ASAT) of ≤2.5 UNL or ≤5 UNL in case of liver metastasis. * Life expectancy ≥3 months. * With normal electrocardiogram results and no history of congestive heart failure. * With normal coagulation function: activated partial thromboplastin time (APTT), prothrombin time (PT) and INR, each ≤ 1.5 x ULN. * Female subjects of child-bearing potential must agree to use contraceptive measures starting 1 week before the administration of the first dose of Tislelizumab until 8 weeks after discontinuing study drug. Male subjects must agree to use contraceptive measures during the study and 8 weeks after last dose of study drug. * With written informed consent signed voluntarily by patients themselves or their supervisors witted by doctors. * Have a known PD-L1 CPS/MMR (or MSI) /HER2(FISH) test result, or have sufficient samples for relevant testing. Exclusion Criteria: * Patients with a history of another neoplastic disease within the past three years, excluding basal cell carcinoma of the skin, cervical carcinoma in situ, or nonmetastatic prostate cancer. * Patients with brain or central nervous system metastases, including leptomeningeal disease. * Pregnant (positive pregnancy test) or breast feeding. * Serious, non-healing wound, ulcer, or bone fracture. * Significant cardiac disease as defined as: unstable angina, New York Heart Association (NYHA) grade II or greater, congestive heart failure, history of myocardial infarction within 6 months Evidence of bleeding diathesis or coagulopathy. * History of a stroke or CVA within 6 months. * Clinically significant peripheral vascular disease. * HIV-positive, active hepatitis B or C (HBV, HCV); * Inability to comply with study and/or follow-up procedures. * Patients with any other medical condition or reason, in that investigator's opinion, makes the patient unstable to participate in a clinical trial. * Her2-positive (IHC 3+ or 2+/FISH+) patients.