All colonoscopy were performed in the afternoon. The day before the colonoscopy: 3 bags of pre-packaged food were used for diet preparation. All the patients drink single dose of 60g Polyethylene Glycol (PEG-4000) with 1L water at a rate of 250 ml every 15 min in the evening.

On the day of the colonoscopy: a bag of pre-packaged food were used for breakfast. All the patients drink single dose of 120g Polyethylene Glycol (PEG-4000) with 2L water 4-6 h before colonoscopy at a rate of 250 ml every 15 min.

All colonoscopy were performed in the afternoon. The day before the colonoscopy: restricted diet prepared by patients were used for diet preparation. All the patients drink single dose of 60g Polyethylene Glycol (PEG-4000) with 1L water at a rate of 250 ml every 15 min in the evening.

On the day of the colonoscopy: restricted diet prepared by patients were used for breakfast. All the patients drink single dose of 120g Polyethylene Glycol (PEG-4000) with 2L water 4-6 h before colonoscopy at a rate of 250 ml every 15 min.

The day before the colonoscopy: 3 bags of pre-packaged food were used for diet preparation.

On the day of the colonoscopy: a bag of pre-packaged food were used for breakfast.

The OBPS，a scoring system with scores between 0 and 14, where 0 is the best score，divides the colon into 3 segments (right, transverse and rectosigmoid colon); each is scored from 0-4 (0=excellent, If it is necessary to suction liquid stool to adequately see the colonic wall, a score of 2 (fair) is given; if it is necessary to wash and to suction, a score of 3 (poor) is given. The remaining scores fall into place around these anchors. ). A score 0-2 is added to indicate the total amount of luminal fluid.(0=The total amount of fluid in the colon is very small,1=a moderate volume of fluid overall in the colon,2=a large volume of fluid overall in the colon)