dMMR/MSI-H colorectal cancer patients are the dominant population of immunotherapy/neoadjuvant immunotherapy, but imaging evaluation of immunotherapy efficacy is insufficient. There are some cases, although no disease remission was found on imaging,pathological complete response (pCR) was confirmed after surgery. Meanwhile,previous studies have shown that dynamic changes in ctDNA can help assess immunotherapy efficacy. Therefore, we propose to conduct a multicenter, prospective, observational clinical study to explore the efficacy prediction and monitoring value of ctDNA in immunotherapy for advanced or locally advanced dMMR/MSI-H colorectal cancer.