Abstract: The insulated gate bipolar transistor (IGBT) full-bridge module (6 in 1) for electric vehicles has the characteristics of high-power density and compact structure. Transient thermal impedance test is a necessary means to establish the thermal network model of IGBT full-bridge module. The method of heating IGBT or FRD separately to test transient thermal impedance does not consider the thermal coupling effect of IGBT full-bridge module in actual working condition, and thermal coupling will largely affect the accuracy of junction temperature evaluation based on thermal network model. The finite element simulation model of full-bridge module for electric vehicle was established, and the thermal analysis of chips and chips, chips and arms and phase to phase thermal coupling was carried out. At the same time, the influence of thermal coupling on the transient thermal impedance test results was analyzed, and the thermal conductivity characteristics of the module were compared with the actual working conditions. The results show that the thermal coupling between chips in the same bridge arm is the stronger, and the thermal coupling between chips and bridge arm and between phases is weak. Finally, combined with the actual working condition, the heat distribution close to the actual working condition was proposed.