With the application of finite element method (FEM), the spindle model of horizontal boring and milling machining center was established. The thermal characteristics of the spindle model was analyzed, and the model was gridding and partitioned by using ANSYS Workbench via calculating heat transfer coefficient of the model, confirming thermal boundary conditions and calculating results of thermal power of spindle bearing of machine tool under different rotational speed. The results show that the heat generation of the No3 bearing is quite large, which has greater influence on the temperature distribution of the spindle. In the design process of machine tool spindle, the machining accuracy can be optimized by consideration of improving the cooling condition and thermal error compensation methods to control the temperature of spindle. The temperature of the bearing is raised with the spindle speed increasing, as well as the system temperature of thermal equilibrium, so more time was taken to get to the thermal equilibrium. The analysis results provide references for the design of spindle of machine tool and structural optimization.