Objective To compare the efficacy of different doses of Polyethylene glycol electrolyte powder (PEG) on the quality of bowel preparation in patients with constipation. Methods Totally 55 cases were involved in this study and divided into two groups: group A in which patients received 2 L whole-dose PEG (n = 26) and group Bin which patients received 4 L spilt-dose PEG (n = 29). All patients received colonoscopy afterwards. Boston bowel preparation scale (BBPS) was used to assess the intestinal cleanliness. The adverse reactions and evaluation of drug taste were recorded. Results Statistically significant difference in BBPS total score, and score in the left colon, the middle colon and the right colon were witnessed in two groups (P < 0.05). No obvious difference was observed between two groups in terms of colonic polyp detection rate, adverse reaction and evaluation of drug taste (P > 0.05). Hierarchical analysis revealed that the BBPS score in the total and the right colon were higher in female patients of group B than that of male patients (P < 0.05). There was no statistically significant difference in BBPS score between the male and the female in group A (P > 0.05). History of abdominal surgery exerted no significant impact on BBPS score in both groups (P > 0.05). Conclusions 4 L spilt-dose PEG is a better option for chronic constipation patients.