The evening before the colonoscopy: The patients who are randomized to the " PEG-Mizone prep " arm will drink single dose of 60g Polyethylene Glycol (PEG-4000) with 0.6L Mizone+0.4L water at a rate of 250 ml every 15 min.

On the day of the colonoscopy: The patients who are randomized to the " PEG-Mizone prep " arm will drink single dose of 120g Polyethylene Glycol (PEG-4000) with 1.2L Mizone+0.8L water 4-6 h before colonoscopy at a rate of 250 ml every 15 min.

The evening before the colonoscopy: In the PEG-ELS prep group, all the patients drink single dose of 60g Polyethylene Glycol (PEG-4000) with 1L water at a rate of 250 ml every 15 min.

On the day of the colonoscopy: 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 evening before the colonoscopy:60gPEG-4000 was mixed with 0.6LMizone and 0.4L water.

On the day of the colonoscopy:4-6 hours before colonoscopy, 120gPEG-4000 was mixed with 1.2LMizone and 0.8L water.

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)

Including Electrolyte disorders requiring clinical treatment, etc.

On the day of the patient's colonoscopy, each patient will complete another form regarding their bowel preparation experience with a modified 5-point Likert scale for ease of use (score 5-1: very easy, easy, tolerable, difficult or very difficult).