The objective is to investigate the efficacy and safety of Envafolimab combined with chemotherapy in the treatment of metastatic or recurrent gastric adenocarcinoma.

Envafolimab indication: Envafolimab is the world's first subcutaneous injection of PD-L1 monoclonal antibody.Suitable for adult patients with advanced solid tumors with unresectable or metastatic microsatellite highly unstable (MSI-H) or mismatch repair gene defects (dMMR).

Inclusion Criteria:

• 18-75 years；
• ECOG 0-1；
• life expectancy of at least 3 months；
• Negative for HER2 gene expression by central laboratory ；
• The tumor expresses PD-L1 as detected by the central laboratory, and the tumor proportion score (CPS) ≥ 1；
• At least one measurable objective tumor lesion by spiral CT examination, the maximum diameter ≥ 1cm（according to RECIST 1.1）；
• Diagnosed by histology and/or cytology, and assessed by imaging (refer to RECIST 1.1) as advanced metastatic gastric adenocarcinoma or gastric adenocarcinoma that has recurred after previous gastric cancer surgery;
• Not received systemic chemotherapy in the past. Patients who have previously received fluorouracil monotherapy or fluorouracil-based adjuvant therapy or neoadjuvant therapy, and patients who have completed adjuvant therapy or neoadjuvant therapy before the start of the chemotherapy regimen in this trial 6 months ago can be included in this trial. In cases of metastatic disease requiring local remission, remission therapy with radiosensitizing doses of fluorouracil or capecitabine alone is permitted only ；
• satisfactory main organ function，laboratory test must meet the following criteria: hemoglobin (HGB) ≥90g/L, neutrophil count（ANC） ≥1.5×109/L, platelet count（PLT） ≥80×109/L, Serum creatinine（CR）≤1.5 upper normal limitation (UNL)，total bilirubin (TBil) ≤1.5 upper normal limitation (UNL), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤2.5 UNL (For patients with liver metastasis, the AST/ALT must be ≤5.0 UNL), Activated partial thromboplastin time (APTT), international normalized ratio (INR), prothrombin time (PT) ≤ 1.5×ULN; left ventricular ejection fraction (LVEF) ≥ 50%; thyroid stimulating hormone (TSH) within the normal range Within: if the baseline TSH exceeds the normal range, subjects with total T3 (or FT3) and FT4 within the normal range can also be enrolled;
• Subjects of childbearing potential must use an appropriate method of contraception during the study period and within 120 days after the end of the study, have a negative serum pregnancy test within 7 days prior to study enrollment, and must be non-lactating subjects ；

Exclusion Criteria:

• Suffered from other malignant tumors within 5 years before the start of treatment in this study;
• Pathologically suggested patients with abnormally increased AFP OR MSI-H ；
• Grade ≥1 unresolved toxicities (according to the most recent version of the National Cancer Institute [NCI] Common Terminology Criteria for Adverse Events [CTCAE]) due to any prior therapy, excluding alopecia and fatigue; neurotoxicity was Recovery to ≤ grade 1 or baseline before the group;
• Subjects with any severe and/or uncontrolled disease ；
• Poorly controlled diabetes (fasting blood glucose [FBG] > 10 mmol/L) ；
• Received major surgical treatment, incisional biopsy, or significant traumatic injury within 28 days prior to the start of study treatment; or had a long-term unhealed wound or fracture；
• Serious arterial/venous thrombotic event within 6 months prior to initiation of study treatment ；
• Previously received drug therapy against PD-1, PD-L1 and other related immune checkpoints ；
• Participating in or participating in other clinical investigators within 4 weeks prior to the start of the study ；
• Allergic to the active ingredients or excipients of the study drug ；
• Unsuitable for the study or other chemotherapy determined by investigator.