Preparations before colonoscopy usually include three preparations, such as understanding the basic conditions, eliminating infectious diseases and cleansing the intestines. 1. Understanding the basic condition: before doing colonoscopy, we should know whether the patient is suitable for doing colonoscopy and whether he/she suffers from other cardiac function diseases, so as to prevent the patient from inducing myocardial ischemia, angina pectoris and other diseases in the process of doing colonoscopy. 2. Excluding infectious diseases: before doing colonoscopy, it is necessary to carry out the examination of infectious diseases to check whether the patient has infectious viruses such as hepatitis B and syphilis, etc. After knowing clearly whether the patient has infectious diseases, it is necessary to check whether the patient has infectious diseases. After knowing clearly, the instrument should be sterilized more strictly for patients with infectious diseases, and at the same time, it should be differentiated from the mirrors used by ordinary patients to prevent cross-infection. 3. Clean the intestines: clean the intestines is more important, patients generally need to try to take some semi-fluid food and low residue food three days before the colonoscopy, in addition to the night before and the day of the colonoscopy need to take oral polyethylene glycol electrolyte dispersions, the intestinal feces in the bowel clean, to avoid leakage of diagnosis due to the intestinal tract is not clean. For patients undergoing gastroenteroscopy, they should strictly follow the doctor’s instructions to make good preparations to prevent misdiagnosis and leakage of diagnosis.