High quality bowel preparation is essential for successful colonoscopy. This study aimed to assess the impact of the dose of Polyethylene Glycol(PEG) on the quality of bowel preparation. This prospective, endoscopist-blinded, randomized, controlled study was conducted. 3L-group received 3 bags of PEG and were instructed to drink 1L at 19:00 in the evening before the day of colonoscopy at a rate of 250 mL every 15 minutes, and to drink the remaining 2L 4 to 6 h before colonoscopy at the same rate. 4L-group received 4bags of PEG and were instructed to drink 2 L at 19:00 in the evening before the day of colonoscopy at a rate of 250 mL every 15 minutes, and to drink the remaining 2L 4 to 6 h before colonoscopy at the same rate. The primary outcome was the quality of the bowel preparation according to the Boston Bowel Preparation Scale (BBPS). The secondary outcomes included polyp detection rate (PDR), adenoma detection rate (ADR), tolerance, and subjective feelings of patients.

Inclusion Criteria:

1. Outpatients aged 18-75 years
2. Referred for routine diagnostic colonoscopy
3. Patients who have signed inform consent form

Exclusion Criteria:

1. pregnancy or breastfeeding
2. allergy to purgatives
3. suspected intestinal obstruction, stricture, or perforation
4. hemodynamic instability
5. impaired swallowing reflex or mental status
6. severely medical status, such as New York Heart Association grade III or grade IV congestive heart failure and severe renal failure
7. history of oesophagus、stomach 、duodenum、small intestine or colorectal surgery
8. participation declined.