A three-dimensional (3-D) finite-difference time-domain (FDTD) method is developed for simulating the lightning-radiated electromagnetic field over the two-dimensional (2-D) rough ground. This 3-D FDTD method provides a new approach to analyze the propagation effect of 2-D rough ground on the lightning horizontal field. It is noted that the effect of the 2-D surface roughness on the horizontal field cannot be ignored even at a distance of 100 m from the lightning channel, and the increase of the land roughness results in a lower magnitude of the horizontal field waveform. When the root-mean-square height (RMSH) of the rough surface is 5 m, the magnitude of the horizontal field at a height of h = 10 m above the rough ground reduces to about half of its field peak value with respect to the smooth ground within distances of 100 m from the lightning channel. We also found that the horizontal field is more obviously affected by the land roughness when the land conductivity is smaller than 0.001S/m.