ObjectiveTo explore a better segment of fixation and fusion for high-grade spondylolisthesis. MethodsA total of 21 patients with high-grade spondylolisthesis who had undergone reduction and posterior instrumented fusion between July 2007 and March 2012, were retrospectively reviewed. All cases underwent posterior spinal canal decompression, Schanz screws fixation and reduction, and intervertebral and posterolateral fusion. The concept of "unstable zone" and the feature of spinal deformity helped us to identify the most appropriate segment to fuse. The pre/post-operative differences on slip percentage, pelvic incidence (PI) and lumbosacral angle were compared and analyzed. The nerve function was evaluated by physical examination and neurological Frankel grade. The Visual Analogue Scale (VAS) and Oswestry Disability Index were used to assess clinical and functional outcomes of lower limbs. Bone fusion was assessed using CT reconstruction. ResultsAll patients were followed up between 12 and 48 months. The clinical and radiological outcomes such as VAS scores and PI angle were all improved compared with that of preoperative, and the differences were all statistically significant (P<0.05). ConclusionFor children with severe spondylolisthesis, if not combined with structural scoliosis, the fixation and fusion level should be up to the upper vertebra which PI angle>60°, and try to protect the posterior longitudinal ligament complex in case adjacent segments become instability or even slip. For adults with severe spondylolisthesis, if not combined with other spinal disorders such as severe osteoporosis, only mono-segmental fusion is recommended after reduction. If the slipped vertebrae could not be reduced to Meyerding gradeⅠ, two or more segments would need to be fixed and fused.