Objective To investigate the effects of simethicone administered at different time on bowel preparation before colonoscopy. Methods A total 270 patients undergoing colonoscopy were randomly divided into three groups and all patients used polyethylene glycol electrolyte solution (PEG-ELS) for bowel preparation. Group A: the patients were administered with 30ml simethicone 2 h before colonoscopy; group B: the patients were administered with 30ml simethicone 1 h before colonoscopy; group C, the patients were administered with 30ml simethicone 0.5 h before colonoscopy. The bowel preparation quality, the bubbles in colon lumen, polyp detection rate and operating time were observed. Results There is no significant difference in bowel preparation quality among the three groups (P>0.05). Compared with group A and C, group B had less bubbles in colon lumen, higher polyp detection rate and shorter operating time (P<0.05). There were no significant difference in bubbles in colon lumen, polyp detection rate and operating time between group A and group C (P>0.05). Conclusion Administration of 30ml simethicone 1 h before colonoscopy could reduce bubbles in colon lumen, improve the detection rate of polyps and shorten the operating time.