The invention discloses an overflow quantity calculation method for a rubber dam. The water flow and flow information of the top end of a dam bag is collected at regular time by determining the width of the dam bag; the section area is calculated according to the width of the dam bag and the water level difference and the flow speed difference during two times of collecting time; and the flow in the collecting periods is calculated according to the section area and the collection interval, the flow of all the collecting intervals is accumulated, and the overflow quantity of the rubber dam is obtained. According to the overflow quantity calculation method for the rubber dam, effective measurement can be conducted according to data of the overflow quantity flowing through the dam bag in a river channel, and effective bases and reference are provided for flood prevention and farm irrigation.