Objective To evaluate the effectiveness of lumbopelvic fixation using the combination of closed multi-axial screws (CMAS) iliosacral fixation system and the posterior segmental spinal fixation for unstable sacral fractures. Methods Between January 2013 and November 2014, 25 patients (39 sides) with unstable sacral fractures were treated with lumbopelvic fixation using the combination of CMAS iliosacral fixation system and the posterior segmental spinal fixation. There were 17 males and 8 females, aged 19-55 years (mean, 33.9 years). The causes were traffic accident injury in 15 cases, falling injury from height in 8 cases, and crushing injury in 2 cases. The interval of injury and operation was 1-13 days (mean, 3.5 days). Fracture was classified as Denis type I in 2 sides, type II in 20 sides, and type III in 17 sides; nerve injury was rated as Gibbons grade I in 2 cases, grade II in 2 cases, grade III in 7 cases, and grade IV in 9 cases. The reduction quality was evaluated by Matta criterion, the clinical function outcome by Majeed, and nerve function by Gibbons criterion. Results The average operation time was 110 minutes (range, 80-150 minutes). The average blood loss was 570 mL (range, 250-1 400 mL). Superficial wound infection occurred in 2 patients, and was cured after debridement and antibiotic therapy. All patients were followed up for an average of 18 months (range, 15-22 months). Postoperative X-ray and CT examination showed clinical healing of sacral fractures at 8-12 weeks after operation (mean, 10 weeks). The mean removal time of internal fixation was 13 months (range, 12-20 months). No screw loosening and fracture, adhesion of internal fixation to surrounding tissue, and obvious electrolysis phenomenon occurred. According to Matta criterion, reduction was rated as excellent in 32 sides, good in 6 sides, fair in 1 side, and the excellent and good rate was 97.5%. According to Majeed functional scoring at last follow-up, the mean score was 84.7 (range, 64-98); the results were excellent in 18 cases, good in 5 cases, and fair in 2 cases, and the excellent and good rate was 92.0%. The nerve function was significantly improved when compared with preoperative one; nerve injury was rated as Gibbons grade I in 8 cases, grade II in 8 cases, grade III in 3 cases, and grade IV in 1 case. Conclusion Lumbopelvic fixation using the combination of CMAS iliosacral fixation system and the posterior segmental spinal fixation is a relatively effective fixation for unstable sacral fractures. Not only is the fracture fixation rigid for early full weight-bearing, but also nerve decompression can be performed which facilitates nerve function recovery.