In order to solve the field calibration problem of rain sensors, a new portable automatic calibrator of rain sensors is developed. In the calibrator, the second class standard glass container is used as standard device, and the digitally controlled constant flow pumps are used to control flowing speed. It has high accuracy of rainfall and rain intensity simulation. The system structure and working principles of the automatic calibrator are described in detail. The technical parameters and main functions are clarified. The multiple parameters of the automatic calibrator were tested by the national center for meteorological metrology and the results show that it satisfies the technical requirements of the field test. The calibration results of the automatic calibrator are compared with the 765 type flow meter. The calibration results of both calibrators indicate that the quality of the automatic calibrator is credible.