The process capability index was used to evaluate the machining consistency of the main rail straightness of the machine tool bed. Multiple linear regression and variance analysis were used to study the factors of clamping torque, temperature and the hardness of guideway surface in the experiment. Conclusions are as follows: different machine tools and different persons have no significant effect on main rail straightness; temperature difference before and after processing and degree of the cutter attrition have little effect on main rail straightness; the difference of moment between the two ends of the rail has some certain influence on the straightness of the main rail; the difference of hardness between the two ends of the rail has significant influence on the straightness of the main rail. It provides a theoretical basis and direction for improving the manufacturing consistency of machine tool bed.