Abstract: The physical model based on heat transfer theory for analyzing the non-stationary thermal field of rotor of retarder in automobile was established. Considering heat convection and radiation the boundary conditions were defined in this paper. The finite element governing equation was derived using Galerkin method. The time differential item has been discreted based on Galerkin format that is stable on any condition. And a new style of time varying step method was used in iteration process. The temperature distribution of rotor was analyzed, and the temperature distribution in axel and radial direction was researched. At last, the temperature distribution in axel direction is measured, and it was founded that the testing and analytical data were uniform approximately. The rotor plate is optimized and the temperature rise and the temperature gradient of the rotor drum can be decreased by this article. So the braking performance stability can be improved by this research.