Objective To investigate the effect and safety of different polyethylene glycol 4000 (PEG)oral regimen on cleanliness of bowel preparation.Methods 280 patients received painness colonoscopy examination were randomly divided into A,B and C groups:group A which drank 1 L of PEG solution the day before colonoscopy and the rest 3 L in the morning at the day of colonoscopy;group B drank 2 L of PEG solution the day before colonoscopy and the rest 2 L in the morning at the day of colonoscopy;group C drank 4 L of PEG solution in the morning at the day of colonoscopy.Bowel cleanliness after taking the drug,stool frequency and adverse events before colonoscopy were observed.Results Group A had better bowel preparation compare with group B (P＜0.05),which was similar to that of group C(P＞0.05);But group A had lower incidence of abdominal distension(abdominal pain),nausea(vomiting) than group C(P＜0.05);and group B had worse sleep quality and more defecate frequency than the other two groups(P＜0.05).Conclusion Drink 1 L of PEG solution the day before colonoscopy and the rest 3 L in the morning at the day of colonoscopy provided a better quality preparation and with less adverse reaction,especially suitable for patients with poor quality of sleep,poor health and severe constipation.