The main spindle system is a key part on the machine tools. The quality of thermal behavior influences the machining accuracy of machine tools directly. In this paper, thermal characteristics of spindle system on the XK717 CNC milling machine are analyzed by combining finite element method with cooling model between spindle system and water tank. Effect rule to thermal characteristics of the main spindle system are obtained. The cooling jacket on the main spindle is improved by the results. This research provides the theoretical fundamentals for optimization design of CNC milling machine.