Anti-PD-1 Antibody Plus Regorafenib in Refractory Microsatellite Stable Metastatic Colorectal Cancer

Inclusion Criteria: * Inclusion Criteria: * 1.Has histologically confirmed unresectable adenocarcinoma of the colon or rectum (all other histological types are excluded). 2\. Have progressed from at least 2 lines of standard treatment,including fluoropyrimidines, irinotecan, oxaliplatin, with or without targeted drugs, like bevacizumab and cetuximab (only for RAS wild-type). 3.Has measurable or non-measurable disease as defined by RECIST version 1.1 4.Is able to swallow oral tablets. 5.Estimated life expectancy ≥12 weeks. 6.Eastern Cooperative Oncology Group performance status (ECOG PS) 0 or 1 7. Has adequate organ function. Exclusion Criteria: * 1.Pregnancy, lactating female or possibility of becoming pregnant during the study. 2.Has not recovered from clinically relevant non-hematologic CTCAE grade ≥ 3 toxicity of previous anticancer therapy (excluding alopecia, and skin pigmentation). 3.Has symptomatic central nervous system metastases that are neurologically unstable or requiring increasing doses of steroids to control CNS disease. 4.Has severe or uncontrolled active acute or chronic infection. 5.Known carriers of HIV antibodies. 6.Confirmed uncontrolled arterial hypertension or uncontrolled or symptomatic arrhythmia.