Objective To evaluate the efficacy and safety of oral magnesium sulfate solution in split doses as bowel preparation in elderly patients undergoing colonoscopy.Methods A total of 368 elderly patients undergoing colonoscopy were enrolled at PLA General Hospital.The patients were randomly divided into magnesium sulfate solution orally in split doses group (group A,n=178) and single dose group (group B,n=190).Parameters including general information,defecation frequency,Boston bowel preparation score (BBPS),detection rate of lesions and adverse reactions.Results The frequency of defecations in group A was (7.6± 1.4),more than that in group B (6.6± 1.5) with statistical significance (P＜0.05).The duration of bowel preparation in group A was (128.6±25.3) min,shorter than that of group B (165.4±29.7) min (P＜0.05).The BBPS in group A was (8.09±0.67),better than that of group B (7.34±0.58) (P＜0.05).The detection rates of intestinal polyps and micropolyps (diameter＜0.5 cm) in group A were 73/178 (41.0％) and 51/178 (28.7％) respectively,compared with 58/190 (30.5％) and 37/190 (19.5％) in group B (both P＜0.05).In group A,8 patients reported adverse reactions as abdominal distension and discomfort.One patient had ST-T abnormality of electrocardiogram (ECG).No nausea or vomiting occurred,yet 2 cases needed enema for inadequate bowel preparation.Twenty-one cases in group B reported adverse events including 7 with nausea and vomiting.There were 13 patients treated with enema.Abnormal ECG was found in 4 patients in group B.The satisfaction rate of group A was 97.8％,higher than that of group B (91.6％) (P＜0.05).Conclusions The effect of bowel preparation of elderly patients with magnesium sulfate solution in split dose has a better tolerance,good cleaning effect and low incidence of adverse reactions.It is an ideal choice for the elderly to prepare colonoscopy.