The aim of the study was to investigate the efficacy of combined application of lactulose oral solution and polyethylene glycol electrolyte powder compared with conventional method in preparing for colonoscopy bowel cleanliness in patients with constipation. Ninety patients, who had constipation and needed to have colonoscopy, were divided into study group and control group with 45 cases in each group, respectively. One day before the colonoscopy, patients in the experimental group were given lactulose oral solution and polyethylene glycol electrolyte powder, whereas the patients in the control group were given oral polyethylene glycol electrolyte powder only. The following parameters were then obtained: time of the first defecation (duration of the time from taking the drugs to the first bowel movement), defecation frequency, completion of bowel cleaning (duration from the first bowel movement to the stool becoming clear), and adverse reaction. Cleansing effect in the study group bowel preparation was significantly better than that in the control group (P < 0.05). Specifically, time of the first defecation and completion of bowel cleaning were statistically shorter in the study group than those in the control group (P < 0.05), and defecation frequency was significantly higher in the study group than that in the control group (P < 0.05). Incidence of adverse reaction, however, was not significantly different between the 2 groups (P > 0.05). Combined application of lactulose oral solution and polyethylene glycol electrolyte powder is superior to the conventional method of polyethylene glycol electrolyte powder alone for colonoscopy bowel preparation in patients with constipation. Therefore, combined clinical application of the 2 compounds is strongly recommended for colonoscopy bowel preparation in patients with constipation.