Objective To study the effect of magnesium citrate combined with compound polyethylene glycol electrolyte powder in bowel preparation before colonoscopy.Methods One hundred and six colonoscopy patients with lower digestive tract symptom were divided into study group (54 cases) and control group (52 cases) by random digits table method,the bowel preparation in study group was magnesium citrate combined with compound polyethylene glycol electrolyte powder,in control group was compound polyethylene glycol electrolyte powder.The lesions detection rate,bowel preparation quality,time of first defecation,number of defecation,time of colonoscopy,results of the electrolyte level and liver and kidney function after medication and adverse reaction were compared between two groups.Results There were no statistical differences in lesions detection rate,time of colonoscopy,results of the electrolyte level and liver and kidney function after medication between two groups (P ＞ 0.05).The total effective rate of bowel preparation quality in study group was significantly higher than that in control group [96.3％(52/54) vs.82.7％ (43/52)],the time of first defecation was significantly shorter than that in control group [(32.5 ± 26.1) rmin vs.(47.2 ± 22.4) min],the number of defecation was significantly much than that in control group [(8.2 ± 2.9) times vs.(6.2 ± 2.6) times],there were statistical differences (P ＜ 0.05).The incidence of nausea in study group was significantly lower than that in control group [22.2％ (12/54) vs.48.1％ (25/52)],there was statistical difference (P ＜ 0.05).There were no statistical differences in the incidences of vomiting,abdominal distention and abdominal pain between two groups (P ＞ 0.05).Conclusion Magnesium citrate combined with compound polyethylene glycol electrolyte powder used in preoperative bowel preparation for colonoscopy has better clinical application value.