Diversion colitis (DC) is a common inflammatory complication in patients with temporary ileostomy after rectal cancer surgery, and no standardized medical treatment exists. This prospective, assessor-blinded, parallel-group, randomized controlled trial evaluated whether autologous fecal microbiota transplantation (auto-FMT) delivered through the diverting stoma ameliorates DC and improves post-reversal outcomes. Sixty-six patients with endoscopically confirmed DC were randomized 1:1 to receive daily auto-FMT (n=33) or saline irrigation (n=33) for four weeks. The primary endpoints are changes from baseline to week 4 in endoscopic (modified Harig score, 0-12) and histopathological (0-9) scores. Secondary endpoints include Wexner incontinence score, quality of life (EORTC QLQ-C30/CR29), inflammatory biomarkers, and safety. The study is designed to test whether auto-FMT produces superior improvements in endoscopic and histopathological severity compared with saline control, and leads to better functional outcomes after stoma reversal.