The nonlinear changes of soil resistivity will be caused when the high power microwave (HPM) propagates in soil, and these changes in turn counteract the propagation process so that the energy transmission efficiency of the HPM is reduced. By analyzing the dynamic ionization process of soil, a HPM soil propagation model is constructed by combining the Maxwell equations. The model is numerically validated by using a finite difference time domain method. The numerical results show the physical processes of the HPM propagation and attenuation in soil and the nonlinear change process of soil resistivity. These numerical results are verified by a theoretical analysis.