According to the problem that the existing production process capability index doesn’t meet the requirement of the product process capability index, an improved method is presented. First, based on the analysis of the manufacturing process of the machine tool VMC850B, the finite element method（FEM）was used to analyze the influence law of fixture parameters on the process consistency during machine tool body finishing process. Contrasted the support position, clamping force and clamping position, the result indicates that the support position is the most important influence factors on processing accuracy and consistency. The validity of the conclusion is verified by both the theoretical analysis and experimental data. Then, in this foundation, a method by constrainting on the support position is presented to improve the process capability. The research methods and conclusions of have some guiding significance for improving the consistency of machine tool body finishing process. 