ObjectiveTo investigate the prevalence of preoperative intestinal dysbacteriosis in colorectal cancer patients and its risk factors.MethodsFrom March 2016 to December 2017, patients who received colorectal cancer surgery in the Department of Gastrointestinal Surgery in West China Hospital were prospectively recruited in the study. Fresh fecal samples were collected on the day of admission for gram-stainsmears. The diet, preexisted diseases, medication, and treatment before admission were investigated by a self-designed semi-structured questionnaire.ResultsSum to 257 patients were recruited, 123 patients (47.9%) of them had normal intestinal bacterial, 48 patients (18.7%) were in preoperative dysbacteriosis grade Ⅰ, 61 patients (23.7%) were in gradeⅡ, 25 patients (9.7%) were in grade Ⅲ. Orderly logistic regression showed that patients who received antibiotics for 30 days or more per year had a higher risk of preoperative dysbacteriosis than those who did not receive antibiotics (OR=3.38, P=0.025). Patients with BMI≥24.0 kg/m2 had a lower risk of dysbacteriosis than patients with BMI<18.5 kg/m2 (BMI 24.0～27.9 kg/m2, OR=0.36, P=0.030; BMI≥28.0 kg/m2, OR=0.23, P=0.032).ConclusionMore than half of colorectal cancer patients have intestinal dysbacteriosis prior to surgery, which may be associated with exposure to antibiotics before admission and low BMI.