Precipitation is a regular project of meteorological observation, which is an important parameter for studying global hydrothermal cycle and climate change. Automatic observation of precipitation is an important part of the future development of meteorology in China. So a new-developed precipitation measurement system based on light blocking principle is introduced. Opposite-type laser optical system is designed and the shading signal of the precipitation particles can be captured correctly. The weak signal detection system based on correlation principle is developed to measure the information of particle velocity and size. The data smoothing algorithm is adopted to reduce the system noise in data signal processing. The system can accurately measure the distribution of raindrop distribution, precipitation amount, precipitation type. Compared with data of other similar instruments, the data of the system has good consistency and can meet the requirement of automatic observation of precipitation.