Objective To investigate the effects of different types of diet and time on quality of bowel preparation and comforts of patients. Methods A total of 600 patients undergoing colonoscopy in the center of digestive endoscopy of a hospital were recruited in the study. They were randomly and evenly divided into 3 groups. For patients in Group A,they were administered a non-slag or low-slag semi-liquid diet for 3 days before the examination and the liquid diet 1 day before the examination. For patients in Group B,a low-residue diet was administered to them for 3 days before the examination. For patients in Group C,a low-residue diet for one day was administered before the examination. Compound polyethylene glycol electrolyte powder was used as bowel cleaner in all 3 groups. The differences in bowel preparation,comforts and satisfaction of patients in three groups were compared. Results The quality of bowel preparation in 3 groups were similar（P>0.05）. Patients in both group B and C showed better outcomes than patients in group A in comfort level（hunger,fatigue and bowel discomfort）（P<0.05）. Conclusion Several kinds of diet commonly used in clinical settings can achieve satisfactory bowel preparation quality. Eating low-residue diet one day before the examination can improve patients’ comforts.