A physical model for the electromagnetic scattering of layered media is established. Radar equation and Snell's law are adopted to calculate the electromagnetic scattering field of multiple layer media. The effects of the electrical properties of the layer, i.e., permittivity and loss tangent on the detection range of the radar system and scattering field are studied. The vehicle-mounted ground penetrating radar (GPR) for the exploration of lunar subsurface is taken as the background. The finite difference time domain method is used to model and calculate the whole vehicle-mounted GPR, and the intensity of the scattering echoes is studied. The simulation results are compared with the theoretical results, and they are found to be in good agreement with each other. Finally some analyses of the results are presented.