An optical precipitation auto-measurement system based on a linear image sensor was designed to reveal the physical mechanism and essence of the precipitation. First, according to physical characteristics of precipitation particles and the demand for precipitation observation,an optical unit and a data acquisition and processing platform for the high speed linear image sensor were designed. As the mass acquisition data were inconvenient for transmission and processing under a continuous operation,the real-time binarization and encoding processing of the linear image were carried out by Field Programmable Gate Array (FPGA). To eliminate the influence of performance excursion of parallel light source and pixel photoelectric response nonuniformity on the setting thresholds and to ensure the measurement precision and coherence,an adaptive threshold setting scheme was proposed and realized based on a MicroBlaze soft-core processor in the FPGA. The comparison experiment with tipping bucket pluviometer based on calibration in a laboratory was carried out. Experimental result indicates that the resolution of precipitation auto-measurement system can achieve 0.1 mm, and the error of cumulative precipitation amount in synchronization observation is about 15%. Obtained results can satisfy the requirements of precipitation auto-measurement for stabilization and high precision.