Inclusion Criteria:

          1. Aigned informed consent

          2. Only patients aged 18-75 years were enrolled

          3. Patients with advanced colorectal cancer diagnosed by pathology and imaging.Note: the
             presence of distant metastases should be confirmed by a CT or MR scan.Bone scan should
             be performed if bone metastases are suspected.Local radiotherapy for pain relief is
             permitted for bone metastases.

          4. Measurable lesions based on Response Evaluation Criteria In Solid Tumors (RECIST) 1.1
             must be present.Local radiotherapy of target lesions is not allowed.

          5. pMMR/MSS advanced colorectal cancer patients with disease progression or intolerance
             to third-line treatment after failure of current standard third-line treatment

          6. ECOG 1 minute or less

          7. Tumor specimens that can be used to detect the status of pd-l1, MSI, b-raf and k-ras
             can be provided.This test requires the patient to provide paraffin embedded biopsy
             specimens or white slices.

          8. White blood cells ≥ 4×109/L, platelets ≥ 100×109/L without transfusion, neutrophil
             absolute value (ANC) ≥ 1.5×109/L without treatment with granulocyte stimulating
             factor, and hemoglobin ≥ 90 g/L.

          9. Bilirubin ≤ 1.5 times of the upper limit of normal value, and cereal grass and cereal
             propyl transaminase ≤ 2.5 times of the upper limit of normal value.

         10. Serum creatinine ≤ 1.5 times the upper limit of normal value, or GFR>45 ml/min

         11. Serum albumin ≥ 25 g/L (2.5g /dL)

         12. INR or APTT ≤ 1.5 times ULN

         13. Hepatitis B/C surface antigen positive patients need to be tested for Hepatitis B /C
             virus DNA quantitative test, only < the upper limit of the normal detection value can
             be included in the group, and long-term use of anti-hb/hc drugs

         14. Drug elution time: 28 days or 5 half-lives from the last drug application.

        Exclusion Criteria:

          1. Allergy to any experimental drug or its excipients, or history of severe allergy, or
             contraindication to the experimental drug

          2. Having a history of autoimmune disease or being active

          3. Previous allogeneic bone marrow transplantation or organ transplantation

          4. Congenital pulmonary fibrosis, drug-induced pneumonia, organized pneumonia, or
             ct-confirmed active pneumonia

          5. HIV positive

          6. Active Hepatitis B /C(Hepatitis B /C viruses have higher quantification than normal)

          7. Active stage tuberculosis

          8. Uncontrolled cancer pain

          9. A live attenuated vaccine was injected within 4 weeks before the study began, or a
             live attenuated vaccine is expected to be injected during the trial or within 5 months
             after the end of the trial

         10. Previous use of immunotherapy, including CTLA4, anti-PD-1, or anti-PD-L1 monoclonal
             antibody

         11. CT indicates lung active inflammation

         12. Systemic administration of glucocorticoids or immunosuppressants within 2 weeks prior
             to the trial.Inhaled corticosteroids and halocorticoids are allowed

         13. Use of hormones is contraindicated

         14. Serious cardiovascular disease, myocardial infection, arteriovenous thrombosis or
             cerebrovascular accident, arrhythmia, unstable angina pectoris within 3 months before
             the trial

         15. Uncontrollable increase in blood pressure or blood sugar

         16. History of other malignancies 5 years ago, except for carcinoma in situ of the cervix,
             non-melanoma skin cancer or stage I uterine cancer

         17. Peripheral neuropathy of grade 2 ≥ NCI CTCAE

         18. Serum albumin less than 2.5g /dL

         19. Uncontrolled or symptomatic hypercalcemia

         20. Infection requiring antibiotics within 14 days prior to trial

         21. Chronic enteritis

         22. Clinically significant active gastrointestinal bleeding

         23. Non-diagnostic surgery within 4 weeks before the trial

         24. Any other disease for which there is evidence that the use of the experimental drug
             needs to be restricted

         25. Participate in other trials within 30 days prior to the trial or plan to participate
             in other trials during the trial

         26. Receive other experimental drugs within 28 days before the start of the trial

         27. Women who are pregnant or lactating, or who plan to become pregnant within 5 months
             after the end of treatment.Women of childbearing age should undergo a blood pregnancy
             test 7 days before the start of the trial

         28. Use of PD-L1 monoclonal antibody or lenalidomide contraindications

         29. MSI-H/dMMR in patients with advanced colorectal cancer