The so-called precancerous lesions are not cancer, but the chance of gastrointestinal cancer is much higher after having these diseases, and some patients will progress to cancer if these precancerous diseases are not actively treated. Therefore, once these diseases are diagnosed, they should be treated positively, with regular follow-up treatment in specialist clinics and early intervention. Precancerous lesions of gastric cancer include: (1) chronic atrophic gastritis, (2) gastric polyps, (3) residual gastritis, (4) pernicious anemia with significant atrophy of gastric body, (5) few gastric ulcers; precancerous lesions of colorectal cancer include: (1) chronic ulcerative colitis, (2) colorectal adenoma, and (3) familial adenomatous polyposis. These precancerous lesions of gastrointestinal tract can be detected by gastroscopy, so early gastroscopy is needed for symptoms such as gastrointestinal discomfort, bloating, nausea, belching, blood in stool, diarrhea, constipation, weight loss and poor appetite. Gastrointestinal polyps need to be treated separately. For inflammatory hyperplastic polyps, regular follow-up is possible, but for adenomatous polyps, especially villous adenomas, wide basal polyps, and polyps larger than 1 cm, early endoscopic or combined endoscopic-laparoscopic resection is needed.