Objective To evaluate the efficacy and safety of lactulose combined with polyethylene glycol for bowel preparation before colonoscopy in patients of different risks. Methods A total of 208 patients undergoing colonoscopy were enrolled, including 108 high-risk and 100 low-risk patients. The high-risk patients were divided into group A (54 taking lactulose + polyethylene glycol) and group B (54 taking polyethylene glycol), and the low-risk patients were divided into group C (49 taking lactulose + polyethylene glycol) and group D (51 taking polyethylene glycol). The Boston bowel preparation score, cecal intubation time, withdrawal time, the detection rate of colonic polyps and adenoma, and the incidence of adverse reactions were observed. Results Among the high-risk patients, the Boston bowel preparation score and adenoma detection rate in group A [（6.35±1.15) scores, 46.3%] were significantly higher than those in group B [（5.76±0.89) scores, 22.2%, both Ｐ＜0.05], and the first defecation interval in group A was significantly shorter than that in group B [(1.20±0.85) h VS (3.29 ± 2.93) h, Ｐ＜0.05]. There was no significant difference in adequate bowel preparation rate, polyp detection rate, frequency of defecation or incidence of adverse reactions between group A and B. In the low-risk patients, the first defecation interval in group C was significantly shorter than that in group D [(1.65 ± 1.35) h VS (3.42 ± 2.64) h, Ｐ＜0.05], and the incidence of adverse reactions was significantly lower than that in group D （44.9% VS 64.7%, Ｐ＜0.05）. There was no significant difference in adequate bowel preparation rate, Boston bowel preparation score, adenoma detection rate, polyp detection rate or frequency of defecation between group C and D. Conclusion For the high-risk patients, the effect of lactulose combined with polyethylene glycol for bowel cleansing is better than that of traditional polyethylene glycol in the improvement of the Boston bowel preparation score, adenoma detection rate, and the first defecation interval. For low-risk patients, lactulose combined with polyethylene glycol regimen has few advantages over traditional polyethylene glycol regimen.