Nab-paclitaxel Plus PD-1 Inhibitor Versus Nab-paclitaxel As Second-line Treatment in Advanced Gastric Cancer

Inclusion Criteria: 1. Age of 18-70 years. 2. Second line treatment for cytological or histological diagnosis of recurrent or metastatic gastric and esophagogastric adenocarcinoma. Disease progression after adjuvant therapy within 6 months is eligible. 3. ECOG performance status of 0-2. 4. Estimated life expectancy of at least 3 months. 5. Bone marrow function: white blood cell count≥3.0×109 /L, absolute neutrophil count(ANC) ≥1.5×109 /L, platelet count(PLT) ≥90×109 /L, hemoglobin(HB) ≥90 g/L. 6. Left ventricular ejection fraction (LVEF) ≥ 50%. 7. Liver and kidney function: Creatinine(Cr)≤1.5 x upper limit of normal range(ULN); alanine glutamate transaminase (ALT) and glutamate transaminase (AST) ≤2.5 x upper limit of normal range (ULN), or ≤5 x upper limit of normal range (ULN)when with hepatic metastases，total bilirubin (TBIL)≤1.5 x upper limit of normal range (ULN), or≤2.5 x upper limit of normal range (ULN) when with Gilbert's syndrome. 8. Any acute, clinically significant treatment-related toxicity caused by previous treatment must have been reduced to less than or equal to grade 1, except hair loss. 9. Able and willing to comply with the study plans in this protocol and sign the informed consent. Exclusion Criteria: 1. uncontrollable infections or have received systematic antibiotic treatment within 72 hours before enrollment. 2. Any abnormal bone marrow hyperplasia or other abnormal hematopoietic function. 3. Have a second malignancy within 5 years prior to registration except for cured carcinoma in situ of cervix uteri, non-melanoma skin cancer. 4. Patients with symptomatic brain metastases. 5. Allergic to the chemotherapy drugs or the materials in this study. 6. Suffering from mental or nervous system disorders and unable to cooperate. 7. Pregnant or nursing female patients. Men and women of reproductive age are unwilling to take reliable contraceptive measures during the study. 8. Active autoimmune disease, history of autoimmune disease, use of corticosteroids or immunosuppressants, or use of hormone replacement therapy, such as thyroxine, insulin, etc. 9. Live vaccine was administered within 30 days before enrolment (injectable seasonal influenza vaccine is allowed as it is inactivated). 10. Patients with other diseases not suitable for enrolment, such as active tuberculosis, hepatitis B (after treatment, hepatitis B virus titer HBV-DNA \<500IU/ml, and liver function is normal, but cannot be combined with hepatitis C), hepatitis C, uncontrolled electrolyte disorders ,pericardial effusion, pleural effusion and abdominal effusion, etc. 11. Have participated in other clinical trials within 30 days prior to this study. 12. History of organ transplantation. 13. Patients that researcher consider cannot sign informed consent or complete the study.