A Phase 1b study of the safety, tolerability and preliminary efficacy of IBI351 combined with cetuximab in the treatment of KRAS G12C mutant metastatic colorectal cancer will be conducted based on recommended dose of IBI351, which consists of combined dose escalation phase and dose expansion phase. After confirming the efficacy and safety of IBI351 combined with cetuximab in Phase Ib, an open-label Phase 3 study of the efficacy and safety of IBI351 combined with cetuximab versus oxaliplatin-based mFOLFOX6 regimen or irinotecan-based FOLFIRI with or without bevacizumab in treatment of KRAS G12C-mutated metastatic colorectal cancer will be conducted.