In this article, the effect of a mountain located on the propagation path of lightning-radiated electromagnetic fields was systematically studied by using the two-dimensional finite-difference time-domain method (2-D FDTD), considering the influence of the return stroke (RS) type, the ground conductivity, the mountain height and the width at the base, its position, and the observation distance. The field peak reduction and the wave arrival time delay, compared with those for the perfectly conducting flat ground, are analyzed in detail. The results show that, with an increase in the mountain height, the field peak reduces and the wave arrival time delay increases. The wave arrival time is insensitive to the change of mountain widths. The terrain envelope method can be used to estimate the additional time delay introduced by the mountain. For a mountain lower than 1 km or so, the additional attenuation effect of the mountain on the field peak can be ignored for most practical purposes. Furthermore, the propagation process of the lightning-radiated electromagnetic wave over the mountain was studied by analyzing the spatiotemporal evolution characteristics of the field. The analysis shows that the electromagnetic wave propagates to the back of the mountain through the diffraction process at the mountaintop.