This research protocol outlines an exploratory study on the combination of early-life fecal microbiota transplantation (yFMT) with immunotherapy and chemotherapy in patients with microsatellite stable metastatic colorectal cancer (MSS mCRC). The single-center, single-arm study aims to assess the safety of yFMT in conjunction with immunotherapy and chemotherapy, with a secondary focus on exploring its efficacy and impact on the patients' immune microenvironment. The study will enroll 10 patients aged 18-75 who have progressed after first-line chemotherapy and targeted therapy. The intervention involves six sessions of yFMT every two weeks, alongside PD-1 inhibitor immunotherapy and FOLFIRI chemotherapy. The primary endpoints are the incidence of serious adverse events (SAEs), treatment-related adverse events (TRAEs), and intervention adjustments due to adverse events, while secondary endpoints include progression-free survival (PFS), objective response rate (ORR), and overall survival (OS). The study is expected to last two years from initiation to data analysis completion, and it will be conducted at the Gastrointestinal Tumor Surgery Department of the First Affiliated Hospital of Xiamen University.