The optimal design of impact resistance reduction for grounding devices in mountainous soil structures with a certain slope and vegetation layer thickness is the basic work to improve the lightning resistance of transmission lines to ensure the safe operation of the system. In this paper, a finite element model of the impact characteristics of tower grounding devices in mountainous soil structures is established by combining spatial finite element and time-domain finite difference methods. The soil resistivity is modeled as a function of the electric field strength to simulate its time-varying and spatial distribution during impact dissipation. The thin-layer grounding conductor is equated to a two-dimensional plane with virtual thickness to address the size discrepancy between the grounding conductor’s cross-section and the soil dissipation area. The validation results demonstrate that the FEM established in this paper is effective, and the soil structure indeed has a significant impact on the performance of the grounding device.