The purpose of the phase I/II study is to establish the safety of Combination of Irinotecan and paclitaxel with 5-FU, leucovorin, oxaliplatin and Tislelizumab.

Inclusion Criteria:

1. Patients with advanced unresectable, histologically confirmed adenocarcinoma of the gastric or gastroesophageal junction.
2. With or without measurable lesions.
3. Patients must have a performance status of 0-1 on the Eastern Cooperative Oncology Group (ECOG) scale.
4. Without serious system dysfunction and could tolerate chemotherapy. 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.
5. Life expectancy ≥3 months.
6. With normal electrocardiogram results and no history of congestive heart failure.
7. With normal coagulation function: activated partial thromboplastin time (APTT), prothrombin time (PT) and INR, each ≤ 1.5 x ULN.
8. 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
9. With written informed consent signed voluntarily by patients themselves or their supervisors witted by doctors.
10. With good compliance and agree to accept follow-up of disease progression and adverse events.

Exclusion Criteria:

1. 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.
2. Patients with brain or central nervous system metastases, including leptomeningeal disease.
3. Pregnant (positive pregnancy test) or breast feeding.
4. Serious, non-healing wound, ulcer, or bone fracture.
5. 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.
6. History of a stroke or CVA within 6 months.
7. Clinically significant peripheral vascular disease.
8. Inability to comply with study and/or follow-up procedures.
9. Patients with any other medical condition or reason, in that investigator's opinion, makes the patient unstable to participate in a clinical trial.