Objective To investigate the effect of midazolam pretreatment on perineal pruritus induced by dexamethasone sodium phosphate injection and explore the possible machanisms. Methods Nighty patients scheduled for selective surgery,aged 18-65years,ASA physical status I~II,BMI 18~30kg/m2 were selected in this study.The patients were equally randomized into two groups: midazolam (M) group and normal saline (N) group, 45 cases in each group. Before anesthesia induction, M group received intravenous injection of 0.03mg/kg (1mg/ml) midazolam, while N group received corresponding intravenous normal saline of the same volume. After 1 min, both groups received intravenous injection of dexamethasone sodium phosphate injection of 10mg. The anesthesiologist assessed perineal irritation in both groups and recorded Ramsay scores,the number of cases, visual analogue scale(VAS) scores ,duration of itching and observed the occurrence of adverse reactions such as dizziness, hypotension and respiratory depression in the two groups. Results There was no statistical difference in the two groups.However,compared with the N group, Ramsay scores in the M group were higher (P 0.05),the incidence of pruritus was lower, the degree of pruritus was less and the duration was shorter (P 0.05). Conclusion Midazolam can effectively inhibit dexamethasone sodium phosphate injection-induced perineal pruritus without obvious clinical side effects.