Objective To evaluate the effects of dexmedetomidine as a nerve block adjuvant and intravenous dexmedetomidine infusion in quadratus lumborum block (QLB) for postoperative analgesia with retroperitoneoscopic surgery. Methods Eighty-eight patients scheduled for retroperitoneoscopic surgery were randomly allocated to ropivacaine group (group R, n=30), dexmedetomidine as an adjuvant to ropivacaine group (group RD, n=29) and intravenous dexmedetomidine infusion combined with ropivacaine group (group RDiv, n=29). All patients received QLB before anesthesia induction. Patients in group R and group RDiv were injected with 0.375% ropivacaine 30 ml and those in group RD were injected with 1 μg/kg dexmedetomidine combined with 0.375% ropivacaine 30 ml. Fifteen minutes before the start of the surgery, 1 μg/kg dexmedetomidine 50 ml was infused in group RDiv, 50 ml saline was infused in group R and group RD. All patients received patient-controlled intravenous analgesia (PCIA) after surgery. Effective duration of analgesia, Visual Analogue Scale (VAS) scores at rest and movement 2, 6, 12, 24, 48 h postoperatively, the consumption of propofol and remifentanil used intraoperatively, the consumption of sufentanil and effective pressing times of analgesic pump within 48 h after surgery, the number of patients with remedial analgesia, 15-item Quality of Recovery (QoR-15) scores 48 h after surgery, the time to first exhaust and ambulation after surgery, the length of stay after surgery and the incidence of postoperative adverse reactions were recorded. Results The effective duration of analgesia in group RD was significantly longer than those in group R and group RDiv (P 0.05). Group RD had both lower resting VAS 6, 12 and 24 h postoperatively and moving VAS 2, 6 and 12 h postoperatively than group R (P 0.05), group RDiv had both lower resting VAS 2 and 6 h postoperatively and moving VAS 2 h postoperatively than group R (P 0.05). The consumption of sufentanil and effective pressing times of PCIA within 48 h after surgery in group RD were also lower than those in group R and group RDiv (P 0.05). The QoR-15 scores in group RD were higher than those in group R and group RDiv (P 0.05) 48 h after surgery. The consumption of propofol and remifentanil used intraoperatively, the number of patients with remedial analgesia, the time to first exhaust and ambulation after surgery, the length of stay after surgery and the incidence of adverse reactions in three groups were similar statistically (P 0.05). Conclusion Dexmedetomidine used as a nerve block adjuvant to ropivacaine in QLB can prolong the effective duration of analgesia, reduce postoperative pain and consumption of sufentanil, improve the quality of postoperative recovery and have no effect on the incidence of adverse reactions. Dexmedetomidine infused intravenously along with QLB can reduce postoperative pain with no effect on the effective duration of analgesia, postoperative consumption of sufentanil, quality of postoperative recovery and adverse reactions.