Inclusion Criteria:
1.Be able to understand and voluntarily sign a written informed consent, which must be signed prior to performing the specified study procedure required for the study 2.Age and gender: ≥18 years old and≤75 years old, both men and women. 3.dMMR/MSI-H or POLE/POLD1 gene mutation was confirmed by immunohistochemical PCR or NGS 4.0-1 physical fitness score by the Eastern United States Cancer Collaboration (ECOG)。 5. Expected survival is 12 weeks 6.Radical gastrectomy or enterectomy, including open surgery or laparoscopic surgery, to achieve R0 resection (no residual cancer at the margin) 7.According to the American Joint Committee on Cancer AJCC 8th Edition cancer stage, it was confirmed by histopathology as gastric adenocarcinoma or esophagogastric junction adenocarcinoma or intestinal adenocarcinoma, and the postoperative pathological stage of gastric adenocarcinoma or esophagogastric junction adenocarcinoma was stage III; Patients with postoperative pathological stage of intestinal cancer T1-3N2M0 or T4N+M0 8.No metastasis or recurrence was determined based on images taken after surgery and within the first 28 days of randomization 9. Subjects are required to provide sufficient FFPE tumor tissue specimens or sections for relevant testing 10.The functions of important organs must meet the following requirements:
1. Hematological system（No blood component or cell growth factor was used to support treatment within 7 days prior to the start of study therapy）:
Neutrophil count≥1.5×10\^9/L; Platelet count≥100×10\^9/L; Hemoglobin≥90g/L;
2. Liver function： Serum albumin≥28g/L; Total bilirubin (TBI)≤1.5×ULN; Alanine aminotransferase (ALT)≤3×ULN Aspartate aminotransferase (AST)≤2.5×ULN
3. Renal function:
Serum creatinine ≤1.5×ULN Calculated creatinine clearance≥50 mL/min (using the Cockcroft-Gault formula); Female: CrCl = (140- age in years) × weight in kg × 0.85 72 × serum creatinine in mg/ dL
Male: CrCl = (140- age in years) × weight in kg × 1.00 72 × serum creatinine in mg/ dL
4. Coagulation function:
Subjects not receiving anticoagulation therapy: INR or APTT ≤ 1.5×ULN;
5. Cardiac function: Left ventricular ejection fraction (LVEF)≥ 50 10. Fertile female subjects must undergo a urine or serum pregnancy test within 3 days prior to the first dosing (if the urine pregnancy test result is not confirmed negative, a serum pregnancy test is required, depending on the serum pregnancy result), and the result is negative If a fertile female subject has sex with an unsterilized male partner, the subject must use a highly effective contraceptive method since screening and must consent to continued use of the contraceptive method for 120 days after the last administration of the study drug; Whether to stop contraception after this time point should be discussed with the investigator 11.If an unsterilized male subject has sex with a fertile female partner, the subject must use an effective contraceptive method from the beginning of screening until the 120th day after the last dose; Whether to stop contraception after this time point should be discussed with the investigator.
12.Subjects were willing and able to comply with the schedule for visiting treatment protocol laboratory tests and other study requirements.
Exclusion Criteria:
1. Subjects with other malignancies in the 5 years prior to enrolment do not exclude subjects with other malignancies that have been cured by local treatment, such as basal or skin squamous cell carcinoma superficial bladder cancer cervix or breast carcinoma in situ
2. Have received non-surgical treatment for gastric/esophagogastric junction/bowel cancer (e.g., radiotherapy, chemotherapy and hormone therapy)
3. Liver peritoneum or distant metastasis
4. Inability to take medications orally
5. Unrelieved postoperative complications during screening (such as postoperative infection, suture rupture, gastrointestinal bleeding, pancreatic leakage, etc.)
6. There are chemotherapy drugs contraindicated in this study and any group of adjuvant therapy regiments specified in the regimen cannot be accepted
7. People with active autoimmune diseases that have required systemic treatment within the past two years (such as treatment with disease-modifying drugs, corticosteroids, immunosuppressants) and replacement therapy (such as insulin thyroxine or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) are not considered a systemic treatment
8. There is a pleural, pericardial or abdominal effusion that is clinically symptomatic, requires diuretic treatment and/or requires repeated drainage.
9. There are active or recurrent inflammatory gastrointestinal diseases (such as Crohn's disease ulcerative colitis hemorrhagic enteritis chronic diarrhea, etc.)
10. A history of myocarditis and cardiomyopathy with malignant arrhythmias.Unstable angina pectoris, myocardial infarction, congestive heart failure (grade 2 or higher according to the New York Heart Association Functional Scale), or vascular disease (such as an aortic aneurysm at risk of rupture) requiring hospitalization in the 12 months prior to initial administration,Or other heart damage that may affect the safety evaluation of the investigational drug (e.g., poorly controlled arrhythmias, myocardial ischemia)
11. Medically difficult to control hypertension (systolic blood pressure ≥150mmHg and/or diastolic blood pressure ≥100mmHg) (based on an average of ≥2 measurements).
12. Uncontrolled diabetes.
13. Peripheral neuropathy ≥ grade 2.
14. Severe infection within 4 weeks prior to initial dosing, including but not limited to comorbidized sepsis or severe pneumonia requiring hospitalization; Active infections that have received systemic anti-infective therapy within 2 weeks prior to initial dosing (excluding antiviral therapy for hepatitis B or C)
15. Known active tuberculosis (TB), suspected active TB subjects need to undergo clinical examination to rule out; Known active syphilis infection
16. Subjects with current active hepatitis B (HBsAg positive with more than 2000 copies /ml(500 IU/ml) of HBV-DNA or higher than the lower limit of detection, whichever is higher), for subjects with hepatitis B, are required to receive anti-HBV therapy during the study treatment; Active hepatitis C subjects (HCV antibody positive with HCV-RNA levels above the lower limit of detection)
17. History of immune deficiency; HIV antibody test positive; Systemic corticosteroid hormones or other immunosuppressants are currently being used on a long-term basis
18. Known history of allogeneic organ transplantation and allogeneic hematopoietic stem cell transplantation
19. Prior treatment with PD-1 receptor or its ligand PD-L1 or cytotoxic T lymphocyte-associated protein 4(CTLA-4) receptor
20. Known allergy to any component of any investigational drug; There is a known history of severe hypersensitivity to other monoclonal antibodies
21. Inactivated vaccines are allowed if a live or attenuated vaccine has been administered in the 30 days prior to the first dose, or if a live or attenuated vaccine is planned to be administered during the study
22. Known history of mental illness substance abuse alcohol or drugs
23. Pregnant or lactating women
24. The presence of any past or current abnormality in laboratory tests for treatment of disease that may confuse the study results, affect the subject's full participation in the study, or participation in the study may not be in the subject's best interest
25. Local or systemic disease caused by non-malignant tumors; Or disease or symptoms secondary to the tumor and can lead to higher medical risk and/or uncertainty in the evaluation of survival
26. Any condition that the investigator believes may cause subjects to receive the study drug treatment to be at risk of interfering with the evaluation of the study drug, or affecting the interpretation of the study results
