Objective To investigate the effect of midazolam on cardiopulmonary function during colonoscopy.Methods From June 2014 to June 2015,214 patients underwent colonoscopy were randomly divided into two groups:109 patients in the midazolam group and 105 patients in the non-sedated group(control group).systolic blood pressure(SBP),diastolic blood pressure (DBP),heart rate (HR) and peripheral blood oxygen partial pressure (SpO2) in the two groups before and after colonoscopy were compared.Results In the midazolam group,SBP and DBP decreased more during colonoscopy than in the control group.However,the frequency with a significant change in SBP was similar in both groups.During colonoscopy,HR andSpO2 decreased more in the midazolam group compared to those in the control group.SpO2 levels returned to normal after the procedure in two groups.Conclusion Midazolam has a tolerable effect on cardiopulmonary function and may be safely used during colonoscopy.