Objective To investigate the effect of ultrasound?guided anterior quadratus lumborum block at lateral supra?arcuate ligament on postoperative analgesia and inflammation response in elderly patients undergoing robot?assisted laparoscopic radical prostatectomy. Methods A total of 60 elderly patients who had undergone robot?assisted laparoscopic radical prostatectomy from June 2022 to June 2023 were randomly divided into a group of ultrasound?guided anterior quadratus lumborum block at lateral supra?arcuate ligament combined with general anesthesia （observation group， n = 30） and a general anesthesia group （control group， n = 30）. Both groups received patient?controlled intravenous analgesia after surgery. The first compression time of an analgesic pump and the numbers of effective compression and remedial analgesia were recorded. The VAS scores at postsurgical hours 2， 12， 24， and 48 during rest and coughing were recorded. Interleukin?6 （IL?6） and systemic immunoinflammatory index （SII） at one day before surgery and two hours， one day and three days after surgery were recorded. Anal exhaust time， length of postoperative hospital stay and occurrence of adverse reactions were recorded. Results The observation group， as compared with the control group， had significantly longer first compression time of an analgesic pump and had fewer numbers of effective compressions and remedial analgesic administrations （P < 0.05）. The VAS scores during rest and coughing in the observation group were lower than those in the control group at postsurgical hours 2， 12， 24， and 48（P < 0.05）. As compared with one day before surgery， both IL?6 and SII in the two groups increased at 2 hours， 1， and 3 days after surgery， but the changes in the observation group were lower than those in the control group （P < 0.05）. As compared with the control group， the observation group had shorter anal exhaust time and length of postoperative hospital stay， and a lower incidence of adverse reactions （P < 0.05）. Conclusions Ultrasound?guided anterior quadratus lumborum block at lateral supra?arcuate ligament can provide better postoperative analgesia， reduce inflammatory response and accelerate postoperative recovery in elderly patients undergoing robot?assisted laparoscopic radical prostatectomy.