Objective To investigate clinical results of lower lumbar degenerative instability for the treatment of lumbar posterior pedicle screw system instrumentation combined with bone graft. Methods 38 patinets suffering from lumbar instability were treated with laminectomy ,nerve root canal enlargement and posterior pedicle screw system instrumentate combined with bone graft by pressure.Results All cases were followed up for 7to 13months.According to the system made by Wong xishen and X-ray 13 months after operation,the excellent and good rate of posterior pedicle screw system instrumentate combined with bone graft by pressure was 94.7%,and the stability of fusion lumbar spine reserved. It's fusion rate was 100%.Conclusion Posterior pedicle screw system instrumentate combined with bone graft by pressure is a safe and effective treatment for lumbar instability.