The system comprises a rainfall monitoring module, a self-calibration and rainfall measurement module, a central processing module, a water level monitoring module, and a drainage module. The rainfall monitoring module is configured to monitor rainfall and send a rainfall signal to the central processing module. The self-calibration and rainfall measurement module is configured to transmit ultrasonic signals and receive calibration echo signals to compute the calibrated flight time, and transmit ultrasonic signals to the water surface in the bucket and receive the measured echo signal reflected by the water surface to obtain the measured flight time under the control of the central processing module. The central processing module is configured to receive the rainfall signal to start the water level monitoring module and the self-calibration and rainfall measurement module, which are used to calculate the rainfall value and output it in a fixed format.