An adequate bowel preparation (BP) is essential for a high quality colonoscopy. Patients with ulcerative colitis (UC) show low compliance with BP due to the large volume of lavage solution to be ingested. We sought to evaluate the efficacy of 4 L, 2 L, and 1 L polyethylene glycol (PEG) for BP and identify the related factors of suboptimal BP (SOBP) in patients We conducted a retrospective analysis of UC patients who underwent colonoscopies from January 2017 to March 2022 at our hospital. Quality of BP was documented using the Boston Bowel Preparation Scale (BBPS). BBPS score ≤ 6 is considered SOBP. The related factors associated with SOBP were evaluated using logistic regression analyses. In total, 282 patients with UC were enrolled in our study. The bowel cleansing by BBPS was 8.44±0.84 in 4 L PEG-based BP, 8.29±0.95 in 2 L PEG-based BP, and 7.59±1.17 in 1 L PEG-based BP. On multivariable analysis, extensive colitis (E3), moderate disease activity (mayo score: 6-10) to severe disease activity (mayo score: 11-12), severe endoscopic activity (EMS: 3), biological therapies (infliximab and vedolizumab), and 1 L PEG-based BP were associated with an increased odds of SOBP. Our study demonstrated that 2 L-based and 4 L-based BP is highly effective in UC patients undergoing colonoscopy. Moderate to severe disease activity, severe endoscopic activity, and the use of biological therapies were associated with an increased risk of SOBP in UC patients undergoing colonoscopy.