• 1.The patients voluntarily participated in the study, signed the informed consent, and had good compliance.

• 2\. Age 18-75 years (including 18 and 75) .

• 3\. Metastatic colorectal cancer confirmed histologically and/or cytologically and initially unresectable.

• 4.MSS or pMMR.

• 5.Patients must have at least one measurable lesion (RECIST 1.1).

• 6.ECOG physical condition 0-1 score.

• 7.Expected survival ≥12 weeks.

• 8.Blood examination (no blood transfusion within 14 days, no correction of granulocyte colony stimulating factor or other hematopoietic stimulating factor within 7 days before laboratory examination).

‣ neutrophil absolute value ≥1.5×109/L, platelets ≥100×109/L, hemoglobin concentration ≥9g/dL)

⁃ Liver function test (bilirubin ≤1.5×ULN; Aspartate aminotransferase and glutamic acid aminotransferase ≤2.5×ULN, AST and ALT≤5×ULN in the case of liver metastasis);

⁃ Renal function (serum creatinine ≤1.5×ULN, or creatinine clearance (CCr)≥60ml/min);

⁃ Coagulation, International standardized ratio (INR) ≤1.5×ULN, prothrombin time (PT) and activated partial thromboplastin time (APTT) ≤1.5×ULN;

⁃ Thyroid function, thyroid stimulating hormone (TSH) ≤ the upper limit of normal (ULN); If there is any abnormality, FT3 and FT4 levels should be examined. If FT3 and FT4 levels are normal, they can be selected.

• 9.Reproductive-age women must have a negative serum pregnancy test within 14 days before treatment and be willing to use a medically acceptable effective contraceptive method (e.g., an intrauterine device, oral contraceptives, or condoms) during the study and for 3 months after the last study dose; for male subjects who are married to a reproductive-age woman, surgical sterilization is required or effective contraception is recommended during the study and for 3 months after the last study dose.