Objective To analyze the relationship between the body mass index(BMI)and anal fistula occurrence based on the results of cross-sectional survey.Methods The clinical data of 545 patients with a-nal fistula visiting in the proctology department of this hospital from March 2022 to November 2023 were ana-lyzed.The patients were divided into 4 groups based on BMI:skinny group(BMI＜18.5 kg/m2),normal group(BMI 18.5-＜24.0 kg/m2),overweight group(BMI 24.0-＜28.0 kg/m2)and obese group(BMI ≥28.0 kg/m2).The logistic regression was used to analyze the influencing factors of fistula occurrence.The dose-response relationship between BMI and the occurrence risk of anal fistula was analyzed by combining with the restricted cubic spline(RCS)model.Results The logistic regression analysis results showed that obese was the risk factor of anal fistula occurrence(OR=2.08,95％CI:1.08-3.99,P=0.028).The combining RCS a-nalysis results showed that there was a"J"shaped trend between the BMI and the risk of anal fistula.In addi-tion,smoking could increase the risk of anal fistula occurrence in obese population.Conclusion Overweight and obesity are the risk factors for anal fistula occurrence.It is suggested that the susceptible population should control their body weight reasonably to prevent the occurrence of anal fistula.