A randomized double-blinded controlled trial was performed to explore the association between pre-operative anxiety and intra-operative butorphanol requirement to evaluate the precise sedative requirement and to confirm the sedative effect of butorphanol in patients receiving lower-limb orthopedic surgery. The Amsterdam pre-operative anxiety and information scale and the Ramsay sedation score (RSS) were used to assess the patients' pre-operative anxiety score and sedation state during surgery. Patients were divided into two groups according to their pre-operative anxiety score prior to administration of pre-medication. Patients in each group were randomly divided into a butorphanol group and a 0.9% saline group. A total of 142 patients were enrolled and 131 patients were analyzed. The sedation scores of patients with high pre-operative anxiety in the 0.9% saline group were lower than those in the butorphanol group at each time-point after infusion. An increased pre-operative anxiety score predicted an increased duration to reach an RSS of 4 for an acceptable level of sedation (r(2)=0.887, P<0.0001). In conclusion, butorphanol had a good sedative effect on patients with pre-operative anxiety. The following formula was proposed: Precise dose of butorphanol (mu g/kg)=15.26 + (0.14x pre-operative anxiety score), which may provide an improvement for patients exhibiting a high level of pre-operative anxiety.