Abstract: Insulated gate bipolar transistor (IGBT) power module failure is mainly induced by temperature. In order to improve the reliability of the power module, the RC thermal impedance model was proposed to realize the junction temperature real-time prediction. With the increase of power density, the transverse heat conduction effect becomes outstanding, the thermal coupling effect was serious. There was a large error in the junction temperature calculation for the traditional RC model. In this paper, the physical law that the thermal diffusion angle depends on the heat flux density was revealed. Combined with the Fourier series analytical heat flux model for the multilayer package structure, the relationship between the 1-D RC thermal network and the 3-D heat flux field was established. The diffusion angle based thermal network considering the multi-chip thermal coupling was constructed, which realized the description of the multi-chip dynamic thermal characteristics. The physical principle of thermal diffusion effect and thermal coupling effect on the junction temperature field was revealed. Compared with traditional thermal diffusion angle models, the junction temperature results of the proposed method had the highest accuracy. Finally, the power module SEMIX603GB12E4P was tested to verify the proposed model. The simulation and experimental results demonstrate that the proposed model characterizes accurately the dynamic thermal process under different working conditions, which validates the effectiveness and accuracy of the proposed modeling method. The error is less than 4%, and the conclusion that the accuracy of the proposed model is 16.72% higher than that of the traditional model is validated.