Abstract: In this paper, we present the computed electromagnetic field properties of lightning irradiation on top of buildings over a complex mountainous terrain, using a two-dimensional finite difference time-domain method in cylindrical coordinates. Analyze the effects of lightning-radiated electromagnetic fields on the top of building over a cone-shaped mountain and over real terrain, respectively. It is found that compared with the flat ground, the mountain inclination angle and building height have a significant effect on the lightning-radiated electromagnetic field. The peak values of the vertical electric and azimuthal magnetic fields become larger as the inclination angle of the mountain increases. The peak value of the vertical electric field increases linearly, and the ratios range from 4 to 8 times; as the building height increases, the peak value of the vertical electric field increases logarithmically, and the ratios range from 1 to 1.5 times when the building height is less than 100m; as the distance between the return stroke of the lightning and the mountain increases further, the increment of the vertical electric field peak first increases and then decreases due to the increase of the building height; taking the radar tower standing on Dajian Mountain in Qingdao as an example, the peak value of the vertical electric field at the top of the building is significantly larger than that of the ideal flat ground, which is increased by 3.8 times, however, there is a little effect on azimuthal magnetic field.