The laser beam transmitting in the rain would be scattered and attenuated by the raindrops, so that the laser ranging system will be false alert and invalidation. Based on geometric optics theory, the raindrop backscattering model was founded, and the backscattering coefficients of different rainfall rates were calculated. Combined with engineering application, the signal-to-noise ratio at different rainfall rates and transmission distances were calculated, and the affect of raindrop backscattering to the system SNR is analyzed. The calculation results indicate that the system SNR is decreasing with the increasing of the rainfall rates and the attenuation rate is up to 11% in the heavy rain. And at the same rainfall rate, the attenuation rate of SNR is up to 46% or more when the distance increases from10 m to 15 m. The rainfall rate and distance are the important factors to affect the system signal-to-noise ratio, so the study on backscattering characteristics has important instruction for the design and optimization of high anti-interfering performance laser ranging system.