Objective To evaluate the efficacy of the “Shamrock method”, ultrasound combined with stimulator guided continuous lumbar plexus nerve block for postoperative analgesia in the elderly after total knee arthroplasty by compared with continuous femoral nerve block. Methods An open, randomized, controlled trial was conducted on 80 ASA Ⅱ or Ⅲ patients (65 to 87 years old) undergoing total knee arthroplasty in our hospital from June to December 2014. The patients were randomly assigned to continuous lumbar plexus nerve block group (L group) and continuous femoral nerve block group (F group). The patients of the former group received continuous lumbar plexus nerve block by ultrasound guided “shamrock” and electronic nerve stimulator, and those of the latter group received continuous femoral nerve block by ultrasound guiding combined with stimulator. After the nerve block catheters were inserted and 30ml 0.2% ropivacaine was given, all patients received patient-controlled analgesia (PCA) after the surgery with 0.2% ropivacaine 5ml/h as the background dose. Visual Analogue Scale (VAS) was employed to evaluate the pain level at 6, 12, 24 and 48h, postoperatively. VAS pain scores were recorded at 24 and 48h, postoperatively, during functional excise and muscle strength. The toxic reaction, nausea and vomiting, and nerve damage were recorded after surgery. Results The VAS scores during rest and functional excise were significantly higher in F group than in L group (P＜0.05), but there was no difference in the muscle strength between the 2 groups (P＞0.05). No local anesthetic toxicity or nerve damage was founded in the both groups, and there was no statistical difference in the incidence of side effect such as nausea and vomiting during postoperative analgesia. Conclusion The ultrasound-guided “shamrock” combined with stimulator for continuous lumbar plexus nerve block is superior to continuous femoral nerve block in analgesia for postoperative analgesia in the elderly after total knee arthroplasty.