OBJECTIVE：To inves tigate the application effect and safety of modified compound polyethylene glycol electrolyte powder（SF-PEG）in intestinal preparation method for patients with constipation undergoing colonoscopy. METHODS ：From Jan. 2020 to Aug. 2020，160 patients with constipation undergoing colonoscopy in anorectal department of the affiliated hospital of North Sichuan Medical College were randomly divided into observation group and control group ，with 80 cases in each group. The patients in the control group took 2 boxes of SF-PEG （mixed into 3 000 mL warm water ）orally at 3：00 a.m. on the day of examination，and finished within 2 hours. The patients in the observation group took 1 box of SF-PEG （mixed into 1 500 mL warm water）at 20：00 the night before the examination and 5：00 in the morning of the day of examination ，and drank it within 1 hour；1 bottle of Dimethylsiloxane powder （mixed into 75 mL warm water ）was taken orally at 7：00 a.m. on the inspection day. The satisfaction of intestinal preparation ，intestinal bubble score ，operation time of colonoscopy ，detection of polyps ，tolerance rate ， acceptance rate of repeating intestinal preparation of patients and incidence of adverse reactions were compared between the two groups. RESULTS ：The patients of 2 groups successfully completed bowel preparation and colonoscopy. The intestinal preparation total satisfaction rate ，intestinal bubble satisfaction rate ，polyp detection rate ，tolerance rate and repeating intestinal preparation acceptance rate of patients of observation group were significantly higher than control group （P＜0.05），while the incidence of nausea and vomiting and total incidence of adverse reactions were significantly lower than control group （P＜0.05），and the operation time of colonoscopy was significantly shorter than control group （P＜0.05）. CONCLUSIONS ：Oral administration of SF-PEG in batches combined with Dimethylsiloxane powder for bowel preparation in patients with constipation before colonoscopy have the advantages of high satisfaction with bowel preparation ，less intestinal bubbles ，short operation time ，high detection rate of polyps，high tolerance rate of patients ，high acceptance rate of repeating intestinal preparation and less adverse reactions. It is a safe and effect method for intestinal preparation.