Background: The therapeutic efficacy of second-line chemotherapy for metastatic colorectal cancer (mCRC) is low. Immunotherapy is suitable for MSI-H mCRC, but the efficacy of single-drug immunotherapy is still unsatisfactory. We investigated to combine immunotherapy with bevacizumab and FOLFIRI as second-line treatment for these patients. Methods: This single center exploratory phase 2 study enrolled adults patient with histologically confirmed metastatic colorecal adenocarcinoma, who were failure of oxaliplatin-based chemotherapy as first-line therapy. Patients receive FOLFIRI (iritecan 180mg/m², leucovorin 400mg/m², fluorouracil 400mg/m² bolus, fluorouracil 2400mg/m² 46-48 hours), anti-PD-1 antibody (Nivolumab 240mg, or Pembromab 200mg, or Terriprizumab 240mg), and bevacizumab (5mg/kg). Maintenance therapy was allowed using PD-1, bevacizumab and/or fluorouracil. An-PD-1 antibody is given for up to 24 months. Results: Between November 29,2018 and January 15, 2021, a tolal of 8 patients were enrolled in our study. There were 3 femles and 5 males, and the median age was 39.5 years old. Among 8 patients who were evaluated, complete resonse (CR) was obtained in 3 cases, partial response (PR) was observed in 3 cases, and stable disease (SD) in 2 cases. Objective response rate was 75%, and disease control rate was 100%. The median PFS and the median overal survival have not yet been reached. The most common adverse events (AE) were alopecia (87.5%), nausea (75%), vomit (50%), hypothyroidism (50%), leukopenia (37.5%), hypertension (25%) and proteinuria (25%). Most of the AEs were grade 1 or 2, and the most common grade 3 or 4 treatment-related AE was hypertension (25%). Conclusions: Anti-PD-1 antibody plus bevacizumab and FOLFIRI demonstrated promising activity as second-line treatment for patients with MSI-H metastatic colorectal cancer. Clinical trial information: NCT05035381.