Objective To investigate the clinical effect of lateral rectus abdominis approach combined with presacral decompression for surgical treatment of old Denis type II sacral fractures complicated with upper sacral plexus injury. Methods A retrospective case series study was performed on the clinical data of 9 patients with old Denis type II sacral fractures complicated with upper sacral plexus injury (L₄-S₁) admitted to Zhongnan Hospital of Wuhan University from June 2010 to December 2016. There were 6 males and 3 females, aged (33.1±7.5)years (range, 15-58 years). Embolization of internal iliac artery and preimplantation of abdominal aortic balloon were performed 2 hours before operation under the guidance of digital subtraction angiography （DSA）. Surgery was performed using a single lateral rectus abdominis approach combined with presacral decompression. The operation time, intraoperative blood loss and full weightbearing time were recorded. The visual analogue scale (VAS) and European QOL Five Dimensional health scale (EQ5D) were compared before and after operation. The Gibbons impairment scale was used to assess neurological function. X-ray and CT scan were used to observe internal fixation and fracture healing. The complications during and after operation were recorded. Results The patients were followed up for 24-52 months ［(35.2±5.2)months］. The operation time was (2.9±0.6)hours. The intraoperative bleeding was (573±138)ml, and the full weight-bearing time was (11.6±1.2)weeks. X-ray and CT scan showed bone healing in all patients at the latest follow-up. The VAS and EQ-5D scale improved from preoperative (7.8±0.6)points and (0.34±0.07)points to the final follow-up of (0.8±0.3)points and (0.81±0.05)points (P＜0.05). According to Gibbons classification, 8 patients were grade I  and 1 patient was grade II one year after operation (P＜0.01). Namely, the radiation pain of lower extremities was significantly improved in all patients, among which 8 patients showed pain disappeared and completely returned to normal and 1 patient showed residual numbness and hypoesthesia of the affected limbs. No major complications (eg, iatrogenic lumbosacral plexus injury, vital blood vessels or pelvic organs injury) occurred during the operation. During the follow-up period, only one patient developed traumatic hip arthritis and underwent total hip arthroplasty 6 months after operation. Fractures of the remaining patients were healed well without complications like infection, pressure ulcer or implant failure. Conclusions  For old Denis type II sacral fractures complicated with upper sacral plexus injury, lateral rectus abdominis approach combined with presacral decompression can fully decompress the upper sacral plexus nerve, relieve pain, and promote functional rehabilitation, with low incidence of complications. It is an alternative surgical method for the treatment of old Denis type II sacral fractures complicated with upper sacral plexus injury.