The Green's function of three-dimensional combined-layer soil model is firstly derived using simulated image method. Based on the specified soil model, the influence of soil model on the earth surface potential (ESP) is analyzed for different soil structure configurations. It is found that difference in soil model only changes the absolute value of ESP, but the influence of ESP difference can be ignored under certain conditions. And the physical mechanism for the effect of soil parameter change on ESP is explained theoretically. One of the challenges that have not been solved effectively is how to establish the proper soil model to calculate DC current flowing through the neutral point of transformer accurately. For this challenge, the criteria and threshold value for selecting the soil model reasonably are introduced. Therefore, the arbitrariness and blindness of selecting the soil model can be avoided. The results are useful for calculating accurate results, which can further provide theoretical references for selecting substations near the HVDC ground electrode.