Background: The prognosis of metastatic colorectal cancer (mCRC) patients with RAS gene mutation and microsatellite stable (MSS) is poor. MSS patients account for 95% of CRC and have a low response rate to immunotherapy. Previous studies have reported that chemotherapy and anti-angiogenesis therapy can promote immunotherapy response. This study aims to assess the safety and antitumor activity of sintilimab (IBI308) combined with CapeOx and bevacizumab in the first-line treatment of patients with RAS-mutant and MSS mCRC. Methods: This is an open-label, single-arm, phase II trial. Eligible patients were aged 18 to 75 years with histologically confirmed unresectable metastatic colorectal adenocarcinoma by multidisciplinary team, and had RAS gene mutation and confirmed MSS status. All patients received treatment with sintilimab (200 mg, day 1) plus bevacizumab (7.5mg/kg, day 1) and oxaliplatin (135 mg/m², day 1) and capecitabine (1g/m², bid, day 1-14) of each 21-day cycle. The primary endpoint included objective response rate (ORR) evaluated via RECIST 1.1 criteria and adverse events according to CTCAE 5.0. Secondary endpoint was progression-free survival (PFS). Results: 25 patients were enrolled and received at least two cycle treatment. At baseline, median age was 60 years (range 45-74), 72% of patients were male, ECOG PS 0/1 was 100%, 60% had liver metastases (mets), and the primary tumor site was right-sided colon in 36.0% and left-side colorectum in 64.0%. 25 patients completed at least one efficacy evaluation. 2 (8.0%) patients showed complete response (CR), 19 (76.0%) patients had a partial response (PR) and 4 (16.0%) had stable disease (SD). Patients with liver or lung mets had a higher ORR (93.3% and 100%, respectively) compared to the overall ORR (84.0%). Disease control rate (DCR) reached 100%. Median PFS has not yet reached. No grade 5 adverse events occurred. The most common treatment-related adverse events (TRAEs) in all grades were anemia (19/25, 76.0%), peripheral neurotoxicity (6/25, 24.0%) and neutropenia (17/25, 68.0%). The most frequent grade 3/4 TRAEs were neutropenia (3/25, 12.0%), elevated aspartate transaminase (1/25, 4.0%), elevated alanine transaminase (1/25, 4.0%) and elevated bilirubin (1/25, 4.0%). Conclusions: Combination treatment with sintilimab (IBI308) plus CapeOx and bevacizumab demonstrated a high ORR (84.0%; 93.3% in patients with liver mets, and 100% in patients with lung mets) and a manageable safety profile in RAS-mutant and MSS mCRC. Clinical trial information: NCT05171660.