Precancerous lesions of rectal cancer include: 1. familial adenomatous polyposis: on the basis of polyps leading to the occurrence of rectal cancer; 2. ulcerative colitis: repeated chronic inflammation that causes cell mutations in the process of tissue repair; 3. schistosomal rectal granuloma: in the process of chronic inflammation, cells continuously necroproliferate and produce mutations. Among the precancerous lesions of rectal cancer, polyps have no obvious symptoms, ulcerative colitis and granulomas have related symptoms, common colonoscopy, polyps, ulcer foci or granulomas can be found, and biopsy under colonoscopy can detect cancer cells early.