The invention discloses a dynamic compensation method for an ultrasonic flow meter. The dynamic compensation method comprises the step of measuring time dynamic period compensation and further comprises the step of conducting fluid density compensation, the step of conducting time difference temperature compensation of ultrasonic countercurrent spreading time and ultrasonic fair current spreading time, the step of looking up a calibration coefficient dynamically according to a calibration coefficient lookup table established in advance and the step of calculating the flow of multistage dynamic compensation, and therefore measuring time dynamic period compensation is conducted on the problem that ultrasonic spreading time measurement has deviation due to the water attack effect brought by instant opening and closing of a valve or residual bubbles in a pipeline; furthermore, compensation is conducted on the liquid density change caused by the temperature; temperature compensation is conducted on the time difference benchmark treating the influences of the temperature change of an element in a circuit box, the lookup table is established on the calibration coefficient without a constant value, and dynamic looking-up is conducted during flow calculation.