Motorized spindle is one of the core parts of high-speed machine tool, to a great extent, its thermal characteristics determine the thermal stress and thermal deformations, therefore the research on thermal characteristics is of great significance to increase the accuracy of high-speed machine tool. In this paper, the heat generation developed in the built-in motor and the bearings is calculated. The motorized spindle is modeled and its thermal characteristics analysis by finite element method is done using ANSYS software, in the foundation of analyzing its configuration and heat transfer. The variation regularity of its temperature-rise and temperature field is also summarized. Thereby it provides a powerful theoretical basis for reducing temperature–rise, calculating thermal deformations and improving working conditions.