In hot stamping process, the die suffers complex thermal mechanical loads. In this paper, the process model and the materials model were set up. And with the example of U-channel forming, a coupled thermal-mechanical analysis is performed with ABAQUS. According to the simulation result, the stress and strain fields of the die show a great uneven distribution in place and time. And the explanation for the non-uniform was given out in this paper. Finally, the fatigue life of the die was evaluated, and the suggestions were proved for optimizing the fatigue lifetime of hot stamping die.