The invention discloses a flow calculation method by adopting a river ultrasonic time difference method. The flow calculation method comprises the steps of calculation of coefficients of a flat bottomchannel; calculation of flow velocity of virtual vertical lines; calculation of cross-sectional area of corresponding water channels; calculation of flow by adopting a current meter method. Accordingto the flow calculation method, an ultrasonic time difference sensor is installed in the width direction of a river to measure the water flow velocity, water level and riverbed cross section of a water layer where the sensor is located; the flow coefficients of the flat bottom channel are calculated based on the water level, the riverbed cross section and the sensor installation height, the flowvelocity of the virtual vertical lines is calculated according to the distribution law of the flow velocity along the cross section, and then the cross-sectional flow is calculated according to the current meter method. The flow calculation method calculates the average flow velocity of each virtual vertical line by calculating the coefficients of the flat bottom channel through a virtual verticalline method, and improves the representativeness of the measured flow velocity, so that the test accuracy of a time difference flow meter is improved, and a field comparison test link is eliminated.