Objective To determine the median effective background volume (MEBV₅₀) of 0.2% ropivacaine in ultrasound-guided continuous sciatic nerve block in the elderly patients with knee ankylosing after surgery releasing. Methods Twenty-two elderly patients (aged 65-79 years, weighed 58-90 kg, ASA Ⅰ-Ⅲ) who underwent arthrolysis of knee joint after total knee arthroplasty in our department from January 2014 to May 2016 were recruited in this study. All patients received ultrasound-guided continuous sciatic nerve block by means of subgluteal approach after manipulation with intravenous general anesthesia. After catheter placement, the volume of the injected solution was varied for consecutive patients using the Dixon’s up-and-down stair-case method according to the response of the previous patient (initial volume:5 ml/h; up-and-down steps:0.5 ml/h). We recorded the blockade situation of sensory at 24 and 48 h after the placement of the stimulation catheter. Results For ultrasound-guided continuous sciatic nerve block with subgluteal approach, the MEBV₅₀ of 0.2% ropivacaine was 4.3 ml/h (95% confidence interval:4.0-4.6 ml/h) for analgesia in the elderly after arthrolysis of knee joint. Conclusion When 0.2% ropivacaine is used for ultrasound-guided subgluteal continuous sciatic nerve block in elderly patients after knee arthrolysis, the MEBV₅₀ is 4.3 ml/h.