The discrete particles and their existing space are supposed as a continuous medium, and its equivalent dielectric constant of the composite medium is developed. By using this constant and the attenuations induced by rainfall recommended by ITU, the dielectric constant is completely determined in GHz wave band and its validity is tested. The analytical expression of complex scattering field for a target in rainfall is presented. So the complex differential scattering in rainfall is simulated. The influences of rain, frequency and polarization etc on the scattering field are researched. Computations show that the influence induced by the rainfall on the differential scattering cross section is about 10－3 dB, and the effect produced by the vertical polarization wave is bigger than that resulted by the parallel polarization wave. The obtained result provides a reference in precision guide and target identification.