High-speed permanent magnet synchronous motorized spindle plays a crucial role in high speed grinding which could greatly improve the efficiency of cutting and reduce manufacturing cost. However, for high-speed machining, the spindle system usually generates excessive heat due to its complex structure and the high-speed rotation, resulting in the loss of grinding precision. To control and optimize the heat generation of the spindle, this paper develops a thermal model for the high-speed spindle system, and further analyzes the steady-state thermal property at different grinding speeds. Based on the result of analysis, the temperature of the spindle system is finally optimized with modified cooling system, providing a theoretical basis for the optimization of high-speed permanent magnet synchronous spindle system. After optimization, the temperature of stator reduce significantly to 33°C, which was 53°C before, accordingly the temperature of the rotor is decreased by 19°C.