On the day before colonoscopy, an experienced researcher would go to the ward to have a face-to-face conversation with the patient to know if patients have the risk factors for bowel preparation failure.

The bowel preparation regimens for patients with risk factors would be optimized. In addition to drink single dose of 120g Polyethylene Glycol (PEG-4000) with 3L water 4-6 h before colonoscopy at a rate of 250 ml every 15 min, the patient also drink single dose of 60g Polyethylene Glycol (PEG-4000) with 1L water at 20:00- 21:00 hours on the day before the colonoscopy. Patients without risk factors drink single dose of 120g Polyethylene Glycol (PEG-4000) with 3L water 4-6 h before colonoscopy at a rate of 250 ml every 15 min.

Patients would received a booklet to explain the details of diet restriction, preparation method and the pictures of bowel preparation of results.The researcher would give a detailed oral explanation of the booklet.

Patients in the PEG-nonFFI group would only receive routine patient education on bowel preparation of colonoscopy, which was completed by ward nurse. all the patients drink single dose of 120g Polyethylene Glycol (PEG-4000) with 3L water 4-6 h before colonoscopy at a rate of 250 ml every 15 min.

Patients with risk factor : on the day before the colonoscopy at 20:00- 21:00 hours, 60gPEG-4000 was mixed with 1L water; 4-6 h before colonoscopy, 120gPEG-4000 was mixed with 2L water.

Patient without risk factor: 4-6 hours before colonoscopy, 120gPEG-4000 was mixed with 3L water.

Education completed by experienced researcher.

The BBPS is a validated scoring system with scores between 0 and 9, where 9 is the best score. The score comprises a sub score 0-3 for each colon segment: right, transverse and left colon.

For all participants, BBPS score will be used when colonscopy is withdrawing.

Including nausea, vomiting, abdominal pain and bloating,etc.

The number of colonoscopy reaching the cecal area divided by the total number of colonoscopy