Raindrop spectra describing the variation of precipitation particle concentration with particle size is the most basic micro-physical characteristics parameter. Using the information of raindrop spectra, we'll have a clearer understanding of the development of natural precipitation process. From the relative position that the raindrop is in the rectangular beam, the variety of transmission intensity of rectangle beam with raindrop size were theoretically analyzed, and the results showed a nice linear relationship. It pointed out that the type of precipitation (drizzle, rain, snow, soft hail, hail as well as mixed precipitation) can be obtained by measuring raindrop size and fall time, and the information can be calculated, such as precipitation intensity, amount of precipitation and particle spectrum distribution.