Power semiconductor device IGBT is related to the reliability of the power module, so it is necessary to obtain accurate IGBT junction temperature. In this paper, an electro-thermal model of IGBT is established based on the theory of electro-thermal analogy. The methods of interpolation, curve fitting and cyclic iteration are used to build the model, so junction temperature of IGBT module under different input conditions can be obtained. We also use the model to evaluate the efficiency of water cooling. The model can not only estimate the junction temperature accurately, but also assist the heat sink design, which helps to improve the reliability of the power module and has strong versatility.