We present an improved finite-difference time domain (FDTD) method based on the moving computational domain technique for the calculation of lightning-generated fields over large-scale irregular terrain. In the improved method, a spatial window moving along with the wavefront at the speed of light is defined and, at each time step, the field components are updated only in that window. The time delay of the wavefront arrival caused by the propagation effect and the irregular terrain is considered when determining the width of the moving window. The efficiency and accuracy of the improved model are validated by comparing its results with those calculated by conventional FDTD, considering an example of lightning-radiated electromagnetic field propagation over complex mountainous terrain. The gain in the CPU time can be as high as a factor of 4 to 5, while the accuracy remains unchanged with respect to the conventional FDTD.