Colonoscopy is used to examine the colon for inflammation, ulcers, tumors, lesions due to parasites, and unexplained diarrhea. It can detect highly located masses that cannot be palpated by rectal fingerprinting. Colonoscopy can be used both for diagnosis and as a therapeutic instrument. Colonoscopy can not only examine the size, shape, location, activity and even find early lesions of tumor, but also remove polyps or early tiny cancerous foci, and biopsy the suspicious foci of tumor by directional microscopy. In colorectal cancer screening, it is often used as the “gold standard” for evaluating the effect of various initial screening tests. It is of great significance to the prevention and early detection of colon cancer, so it is the most effective means to diagnose colon cancer at present. Since the incidence of colitis, intestinal polyps, rectal cancer and other intestinal diseases has been on the rise in recent years, it is advocated that the first colonoscopy should be done after 40 years old. Electronic colonoscopy is feasible in all of the following cases without contraindications: (1) blood in the stool of undetermined cause or persistent positive fecal occult blood. (2) Chronic diarrhea of undetermined cause. (3) Barium examination suspected end of the ileum and colon lesions need to be clearly diagnosed. (4) Low intestinal obstruction and abdominal mass can not be excluded intestinal disease (5) Colonic polyp resection, hemostasis, sigmoid colon torsion or intussusception reset (6) After colon cancer surgery, polypectomy after the need for regular endoscopic follow-up. (7)Those who need endoscopic assistance in exploration and treatment during surgery for intestinal diseases. (8)Colorectal tumor census. (9)Those who have chronic enteritis and need regular colonoscopy. (10)Patients with intestinal cancer, in order to know the type of tumor and the scope of lesions, need to have colonoscopy before surgery, so as to facilitate the decision of surgical plan. Regular review is also needed after intestinal surgery in order to observe the changes of the condition.