Improving thermal characteristics is a crucial approach for increasing machining accuracy of NC lathe. Modeling and analysis of thermal characteristics to a high speed NC lathe were implemented by using finite element method in this paper. Based on this, temperature rise characteristic of the whole spindle system was obtained. The thermal deformation of spindle system due to thermal-mechanical coupling was calculated. Layout and parameter were optimized for the radiating plate of spindle box. And this leads to a considerable reduction in the run-out error of spindle head. The optimization result reaches the expected goal.