Objective: To compare the effects of 2L polyethylene glycol (PEG) and 1.5L PEG on the quality of bowel preparation after optimized dietary restrictions. Methods: This study was a prospective randomized controlled trial conducted in three hospitals: the First Affiliated Hospital of Naval Medical University, Huadong Hospital Affiliated of Fudan University, and General Hospital of Northern Theater Command. The eligible subjects who underwent examination and therapeutic colonoscopy at each center were randomly assigned to Group A (specialized medical diet + 2L PEG) and Group B (specialized medical diet + 1.5L PEG) according to the randomization table. One day before the examination, all subjects in both groups adopted a low-residue special medical food as a dietary restriction and received 2L and 1.5L PEG respectively for bowel preparation. The outcomes included the adequate bowel preparation rate, excellent bowel preparation rate, the score of bowel preparation, the completion rate of bowel preparation, the satisfaction rate of subjects, the willingness to repeat, the satisfaction rate of endoscopists, and the incidence of adverse reactions. Results: A total of 117 subjects were included (57 from the Naval Military Medical University and 30 from each of the other two hospitals). There were 60 subjects in Group A and 57 in Group B. There were no significant differences in baseline characteristics between the two groups. The adequate bowel preparation rate [81.7% (49/60) vs. 64.9% (37/57), χ2 = 4.21, P = 0.040] and the satisfaction of physicians [88.3% (53/60) vs. 70.2% (40/57), χ2 = 4.21, P = 0.015] in Group A were significantly higher than those in Group B. There were no significant differences in the excellent bowel preparation rate, bowel preparation score, bowel preparation completion rate, subject satisfaction rate, willingness to repeat, and incidence of adverse reactions [The p-values are all greater than 0.05]. Conclusion: Based on the optimization of dietary restrictions, the 2 L PEG regimen significantly outperformed the 1.5 L PEG regimen, indicating that reducing PEG to 2 liters is more reasonable than reducing it to 1.5 liters.