Inclusion Criteria:  Male or female between 50-75 years old; Able to read, understand and sign informed consent The investigator believes that the subjects can understand the process of the clinical study, are willing and able to complete all study procedures and follow-up visits, and cooperate with the study procedures Outpatients requiring colonoscopy  Exclusion Criteria:  colorectal surgical history Severe colonic stricture or obstructive tumor Obvious gastroparesis or gastric outlet obstruction or intestinal infarction Pregnant or lactating women Dysphagia or impaired swallowing reflex or mental state Intestinal obstruction or perforation is known Severe chronic renal failure（CCR< 30ml/min） Severe congestive heart failure（NYHA ⅢorⅣ） Uncontrolled hypertension( systolic pressure>170mmHg，diastolic pressure>100mmHg) Dehydration or electrolyte disorder Toxic colitis or megacolon Inable to give informed consent Hemodynamic instability