In order to solve the reliability problem of power module working under temperature cycle, the transient temperature distribution model and the transient thermal coupling model of power module under temperature cycle were established based on finite element software, and the vulnerable areas such as chip and glass insulator were analyzed emphatically. The maximum thermal stress of the glass insulator and the maximum temperature of the chip were taken as the optimization objectives, and the power module was optimized by genetic algorithm. The results show that compared with other studies, the power module obtained by transient thermal coupling is more accurate, the chip temperature is 86.03 ℃, and the thermal stress of the glass insulator is 61.27 MPa. The temperature of the chip optimized by genetic algorithm is 81.85 ℃, and the thermal stress of the glass insulator is 37.05 MPa, which meet the reliability requirements. It is proved that the combination of genetic algorithm and simulation can effectively improve the reliability of product design.