Inclusion Criteria:  Signed the informed consent. Age ≥18. Colonic adenocarcinoma confirmed histologically or histopathologically. No residual cancer was confirmed after radical resection of colon cancer. According to the overall postoperative results, T4NX/TXN+ colon cancer was determined according to AJCC/ UICC TNM staging eighth edition. No liver, peritoneum or other distant metastases. MSI-H or POLE/ POLD1 gene mutation was confirmed by PRC or NGS sequencing. ECOG physical status score is 0 or 1.  Appropriate organ function according to the following laboratory test values obtained within 7 days prior to use on Day 1 of Cycle 1:  A. Hemoglobin value ≥9.0g/dL. B. Absolute neutrophil count ≥1,500/mm3 (≥1.5*109/L). C. Platelet count ≥100,000/mm3 (≥100*109/L). D. Total serum bilirubin ≤1.5* upper normal limit (ULN). E. aspartate aminotransferase (AST/SGOT) and alanine aminotransferase (ALT/SGPT) ≤2.5* upper limit of normal value (ULN).  F. Serum creatinine ≤1.5 times * upper limit of normal (ULN) or creatinine clearance ≥50ml/min.  Willing and able to follow research procedures and visit plans.  Exclusion Criteria:  The patient had received non-surgical treatment for colon cancer (e.g., radiation, chemotherapy, and hormone therapy).  Has a serious illness or medical condition, including but not limited to the following:  A. Recurrent in situ or metastatic tumor of any other site is known. B. Systemic active infection (i.e., infection causes body temperature ≥38℃). C. Clinically significant intestinal obstruction, pulmonary fibrosis, renal failure, liver failure, or symptomatic cerebrovascular disease.  D. Severe/unstable angina, New York Heart Association (NYHA) grade III or IV symptomatic congestive heart failure.  E. Gastrointestinal bleeding of clinical significance. F. Known presence of human immunodeficiency virus (HIV) or acquired routine immunodeficiency syndrome (AIDS) -associated disease, or active hepatitis B or C.  Pregnant or lactating women. The researcher did not consider it appropriate to enter the study.