Abstract: ob＜x＞jective To compare the postoperative analgesic effect and side effects of different volume ropivacaine adductor canal(AC) midpoint block under ultrasound positioning on elderly patients after total knee arthroplasty (TKA).Methods Ninety patients undergoing elective unilateral knee replacement were randomly divided into three groups: ropivacaine 10ml (group A), ropivacaine 20ml (group B) and ropivacaine 30ml (Group C), with 30 cases in each group. All patients underwent AC midpoint block under ultrasonic localization after operation. The VAS scores of rest and exercise at 6, 12, 24 and 48 h after operation were recorded; 48 h cumulative dosage of sufentanil and dizocine; The quadriceps femoris muscle strength and active knee range of motion (ROM), as well as the incidence of side effects such as nausea and vomiting. Results Compared with group A, the VAS scores of resting and exercise in group B and C were lower at different time points after operation(P < 0.05); The cumulative consumption of sufentanil and dezocine were reduced 48 hours after operation(P < 0.05), and the incidence of nausea and vomiting were decreased(P < 0.05); ROM increased(P < 0.05). Compared with group C, the muscle strength of quadriceps femoris in groups A and B was higher at different time points after operation (P < 0.05). Conclusions AC midpoint blocked with 20 ml ropivacaine can provide good postoperative analgesia for TKA with little effect on quadriceps muscle strength, promote the recovery of knee function after TKA.