Abstract: Objective To investigate the value of urgent colonoscopy in the diagnosis of severe acute lower gastrointestinal bleeding and the optimal bowel preparation before examination. Methods The clinical data were collected from 188 patients undergoing wither urgent or elective colonoscopy for severe acute lower gastrointestinal bleeding in Nanfang Hospital. Univariate analysis was used to assess the effect of the timing of colonoscopy on the diagnostic rate of hemorrhage, and a multivariate model which stratified bowel cleanliness was used to analyze the impact of bowel cleanliness on the diagnostic rate of urgent colonoscopy. Results Of the 188 patients, 118 underwent urgent colonoscopy and 70 underwent elective colonoscopy examinations. The diagnostic rates were comparable between the two groups (44.1% vs 41.4%, P=0.724), but urgent colonoscopy resulted in a significantly higher diagnostic rate for identifying the bleeding source (32.2% vs 18.6%, P= 0.041). The proportion of the patients taking oral laxatives was significantly lower in urgent colonoscopy group (P<0.001). Oral laxatives versus enema resulted in good, moderate, and poor bowel cleanliness in 63.6% vs 13.5%, 28.6% vs 24.3%, and 7.8% vs 62.2% of the patients (P<0.001). Univariate analysis indicated that good bowel cleanliness was associated with a significantly higher diagnostic rate of colonoscopy than poor bowel cleanliness (P=0.012). Multivariate analysis showed that with good bowel cleanliness, urgent colonoscopy yielded a significantly higher diagnostic rate than elective colonoscopy (P=0.030); subgroup analyses suggested that good bowel cleanliness improved the diagnostic rate of urgent colonoscopy as compared with poor bowel cleanliness (P=0.015). Conclusion In patients with good bowel cleanliness, urgent colonoscopy yields a higher diagnostic rate than elective colonoscopy for severe acute lower gastrointestinal bleeding. Poor bowel cleanliness resulting from bowel preparation by enema significantly lowers the diagnostic performance of urgent colonoscopy. Oral laxatives are recommended over enemas for bowel preparation before urgent colonoscopy when the patients have stable hemodynamics.