Tyrosine Kinase Inhibitor (TKI) + Anti-PD-1 Antibody in TKI-responded Microsatellite Stability/Proficient Mismatch Repair (MSS/pMMR) Metastatic Colorectal Adenocarcinoma.

Inclusion Criteria: * Subjects who voluntarily participated in the study, signed the written informed consent form, and could comply with the protocol of study. * Male or female of age 18-75 years. * Subjects with colorectal adenocarcinoma who were histopathologically confirmed, and with locally advanced (unresectable) or mCRC. * Subjects who underwent standard antitumor therapies (fluorouracil, oxaliplatin, irinotecan were used, with or without administration of bevacizumab and/or cetuximab). * Patients with MSS/pMMR mCRC (immunohistochemistry, polymerase chain reaction or next-generation sequencing can be used). * All adverse reactions associated with drug use or surgery were reduced to grade 0-1 (according to Common Terminology Criteria for Adverse Events (CTCAE) version 5.0) or to a level required by the protocol criteria. * The presence of at least one measurable lesion by computed tomography (CT) or magnetic resonance imaging (MRI). * Eastern Cooperative Oncology Group (ECOG) performance status score of ≤ 1. * Subjects with life expectancy ≥ 12 weeks. * Adequate important organs functions: bone marrow function (neutrophil count ≥ 1.5×10\^9/L; platelet ≥ 80×10\^9/L; hemoglobin ≥ 90 g/L), liver function (serum albumin ≥ 28 g/L; total bilirubin ≤ 1.5×upper limit of normal (ULN); alanine aminotransferase and aspartate aminotransferase ≤ 3×ULN, or ≤ 5×ULN if liver metastases are present), renal function (serum creatinine ≤ 1.5×ULN or creatinine clearance (CrCl) ≥ 40 mL/min, using the Cockcroft-Gault formula; urine protein \< 2+; 24h urinary protein content \< 1.0 g/24h if urinary protein ≥ 2+ ), coagulation function (international normalized ratio or activated partial thromboplastin time ≤ 2×ULN), thyroid function (thyrotropin ≤ 1×ULN). Exclusion Criteria: * Known microsatellite instability high (MSI-H) mCRC. * Participation in another study with intervention or drugs within the past 4 weeks. * Performing surgery and incomplete recovery within the past 4 weeks. * Subjects with active autoimmune diseases or with related history. Subjects with controlled type I diabetes or hypothyroidism with substitution therapy may be included for further screening. * Any conditions requiring corticosteroids (\> 10 mg per day of prednisone or equivalent) or immunosuppressive drugs as systemic treatment within the past 1 week. * Other active malignancy within the past 5 years, except for the cured limited cancer (such as basal cell carcinoma, carcinoma in situ of the prostate or cervix, etc.). * Subjects with history of hepatic encephalopathy or confirmed metastases to central nervous system. * Subjects with non-infectious pneumonia under steroid treatment within the past 6 months. * Suffering from chronic or active infections, fever (≥ 38.5℃) within the past 1 week, or white blood cell count \> 15×10\^9/L), requiring systemic anti-infective treatment at the screening period, except for viral hepatitis. * Subjects with any other abnormal condition that is inconsistent with the study medication, or may increase the risk of the subject，according to investigators' judgment. * Congenital or acquired immunodeficiency (such as human immunodeficiency virus). * Subjects with active hepatitis B virus (HBV) (HBV surface antigen positive and HBV-DNA \> 2000 IU/ml) or hepatitis C virus (HCV) (HCV antibody and HCV-RNA positive). * Subject who received a live attenuated vaccine within the past 4 weeks, or vaccination is planned during anti-PD-1 antibody treatment or within 5 months after the last treatment. * More than mild pericardial effusion, massive pleural or/and peritoneal effusions need puncture and drainage at the screening period. * Subjects with symptomatic heart and cerebrovascular diseases: heart failure (New York Heart Association class III or IV, left ventricular ejection fraction \< 50%), uncontrolled hypertension or arrhythmias, serious cardiovascular and cerebrovascular events (acute coronary syndrome, stroke, thromboembolism, etc.) within the past 6 months. * Known allergy to targeted drugs. * Women being pregnant, or during lactation, or planning to get pregnant during the trial. * Subjects with any other conditions judged by investigators would be excluded.