A new soil model named as 3d combined-layer soil model is firstly proposed. According to the concept of image, the Green function of this soil structure is derived using complex image method. Based on it, the surface potential distribution (SPD) of different soil structure configuration is calculated, moreover, the influence of soil parameters on SPD is analyzed. It can be seen from the results that the variations of the soil parameters such as soil thickness and resistivity can bring different influence to the SPD of the soil, and it is related not only to the distances, but also the angle for the grounding electrode. Therefore, the 3DSM may be more accurate for the calculation of SPD.