Abstract: The six sigma DMAIC model is used to improve the quality of XinH's Z steering shaft, which is an important part of the recirculating ball steering gear. Firstly, through the analysis of SIPOC flowchart and Pareto diagram, it was found that the quality point needing improvement was the out-of-tolerance of grinding outer roundness; Secondly, in view of the analysis clearly improving quality point, in the use of MSA (measurement systems analysis) and process capability analysis and other methods for measuring system based on determine process capability and process, key combination of FTA(Fault tree analysis) and method of general linear model analysis were used to determine the key influence factors for the center hole run out, with the root cause analysis to find out the cause of center hole to beat large hydraulic clamping device for clamping pressure; Finally, the optimal interval of pressure value determined by regression analysis, the comprehensive improvement scheme developed and implemented, and the improvement result monitored. The improvement results showed that the unqualified rate of the improved Z steering shaft decreased from 8% before the improvement to 1.6%, and the unqualified rate was effectively controlled, achieving the established goal of quality improvement.