It is viewed traditionally that the attenuation and scattering of rain have no effect on C-band scatterometer because its wavelength is greater than the diameter of raindrops, so rain effects on C-band scatterometer wind measurement are often ignored. According to the attenuation and the volume backscatter of the scatterometer signal by raindrops and the perturbation of the water surface by rain, in this paper, we derive the radar equation of the rainfall, collect the data from ASCAT backscatter, rain data from PR and wind field from the European Centre for Medium-Rang Weather Foremasts in 2010, and quantitatively analyze the influence of rainfall on the normalized radar backscattering cross section of the C-band scatterometer. Our results show that the attenuation increases as the rain rate and the incidence angle increase, the volume backscatter and the perturbation of the water surface increase with the increase of rain rate and decrease with the increase of incidence angle, and the influence of the perturbation on the wind measurement of the scattermeter is greater than the volume backscatter. In addition, we establish the C-band active microwave radiative transfer model for the rainfall by the radar equation and the collected data. The experimental results indicate that the new model can improve the C-band scatterometer wind measurement accuracy under rainfall conditions.