XELOX +Bev +Tislelizumab for First-line Treatment of MSS/pMMR RAS-mutated mCRC

The goal of this clinical trial is to compare XELOX +Bev +Tislelizumab with standard chemotherapy，in MSS/pMMR-type RAS-mutated metastatic colorectal adenocarcinoma. The main questions it aims to answer are efficacy and safety of the regimen of XELOX +Bev +Tislelizumab. The investigators want to transform ras-mutated colorectal cancer into a "hot tumor" through the combination of anti-vascular therapy and chemotherapy, and then achieve better therapeutic effect through the combination with immunotherapy. Participants will receive the regimen of XELOX +Bev +Tislelizumab.