Abstract: High power microwave (HPM) subaerial propagation is simulated with finite difference time domain (FDTD) method. In the simulation, a mixed absorbing boundary is presented by combining perfectly matched layer (PML) with generalized perfectly matched layer (GPML). The correctness of the model is verified by contrasting the numerical value with the analytic value at level ground, and the scattering properties of HPM is analyzed by the numerical values at rough ground. Theoretical analyses and simulation results show that, diffraction phenomena arising from the edge can be restrained by using this method. The scattering coefficient curve of rough surface is similar to that at level ground in overall trend, however, it has no Brewster angle.