• 1\. Subjects voluntarily join the study and sign an informed consent form; 2. Male or female patients between the ages of 18-75; 3. Adenocarcinoma of the gastric/esophagogastric junction after radical resection was staged at stage IIIB-IIIC; 4. The molecular pathology of gastric cancer must meet at least one of the following three conditions:

‣ Consistent with high-frequency microsatellite instability MSI-H

⁃ Consistent with high tumor mutation load TMB-H (≥10 mutations/megabases)

⁃ High expression of PD-L1 (CPS score ≥5) 5. Expected survival time \> 6 months； 6. ECOG (Eastern US Cooperative Oncology Group) score: 0-1 points; 7. Have not received anti-tumor immunotherapy; 8. Agree to provide archived tumor tissue samples for molecular testing; 9. Have good organ function:

• <!-- -->

‣ Hemoglobin ≥90g/L;

⁃ Absolute neutrophil count ≥1.5×109/ L;

⁃ Platelet count ≥100×109/ L;

⁃ aspartate or alanine aminotransferase ≤ 2.5 times the upper limit;

⁃ alkaline phosphatase ≤ 2.5 times the upper normal limit ; 10. Women of reproductive age who need to use a medically approved contraceptive method (such as an IUD, contraceptive pill, or condom) during the study treatment period and for 3 months after the end of the study treatment period; For men who agreed to use an appropriate method of contraception during the trial and for 3 months after the last dose of the trial drug.

‣ 11\. Women of reproductive age must be negative for serum or urine HCG within 72 hours prior to study entry. And must be non-lactating.