Thsi study is a randomised, parallel-controlled phase II/III trial evaluating FOLFIRI plus bevacizumab with or without QL1706 as second-line therapy for metastatic colorectal cancer. The primary endpoint is PFS. Secondary endpoint includes overall survival, objective response rate, safety profiles, immune-related adverse events, and patient quality of life. Exploratory analyses focus on dynamic shifts in tumour immune microenvironment and biomarkers, aiming to identify predictive signatures for therapeutic efficacy and immune toxicities.