Inclusion Criteria:

• 1. Willing and able to provide written informed consent. 2. Age ≥ 18 years old. 3.Histologically confirmed metastatic colorectal adenocarcinoma with pMMR/MSS status. 4. Eastern Cooperative Oncology Group (ECOG) performance status 0-2. 5.Discontinued first-line oxaliplatin-based doublet chemotherapy for metastatic colorectal cancer due to intolerable toxicities or disease progression; OR developed recurrent metastatic disease within 6 months after the last dose of adjuvant chemotherapy. 6.Presence of evaluable lesions on imaging examinations. 7. Adequate bone marrow, hepatic and renal function as assessed by the following laboratory requirements conducted within 7 days of starting study treatment. 8.Willing and able to comply with study procedures and scheduled visit schedules

Exclusion Criteria:

• 1.Patients complicated with digestive tract diseases including duodenal ulcer, ulcerative colitis, intestinal obstruction, or other conditions judged by the investigator to potentially cause gastrointestinal hemorrhage or perforation; or massive pleural effusion or ascites requiring intervention. 2.Previous or concurrent cancer that is distinct in primary site or histology from colon cancer within 5 years prior to randomization. 3. Radiological evidence of brain metastases. 4. Prior treatment with irinotecan hydrochloride, or prior receipt of PD-1 and/or CTLA-4 immunotherapy in the first-line setting. 5. Autoimmune diseases requiring continuous systemic steroid therapy. 6. History of laparotomy, thoracotomy or intestinal resection within 28 days prior to enrollment; or unhealed wounds (excluding suture wounds from central venous catheter implantation), gastrointestinal ulcers or traumatic fractures. 7. Any of the following events occurring within 12 months before study enrollment: myocardial infarction, severe/unstable angina pectoris, New York Heart Association (NYHA) class ≥ 2 cardiac insufficiency, clinically significant supraventricular or ventricular arrhythmias, and symptomatic congestive heart failure. 8.Confirmed human immunodeficiency virus (HIV) infection or acquired immunodeficiency syndrome (AIDS)-related diseases. 9. Active inflammatory bowel disease or other colorectal diseases causing chronic diarrhea; interstitial lung disease, non-infectious pneumonia, or uncontrolled systemic diseases (e.g., diabetes mellitus, hypertension, pulmonary fibrosis, acute pneumonia, etc.). 10. Breastfeeding or pregnant women; lack of effective contraceptive. 11. Other severe physical or psychiatric illnesses, or laboratory abnormalities that may increase the risks of study participation or interfere with study outcomes; or patients deemed unsuitable for this study by the investigator.