The invention relates to a ultrasonic cleaning method. Firstly, a multi-phase flow is prepared, and is distributed on the surface of an object to be cleaned with a certain flowing speed, wherein the multi-phase flow comprises a continuous phase and a dispersion phase; the continuous phase is liquid; the dispersion phase contains gas; the gas in the multi-phase flow is initially in a micro-bubble state; then, a ultrasonic generator is started to emit ultrasonic waves to the cleaned object; the ultrasonic waves are propagated in the multi-phase flow to reach the cleaned object; and the initial frequency of the ultrasonic waves is adjusted as FORMULA, so that the multi-phase flow forms a ultrasonic retardant layer with higher resistance on the surface of the cleaned object. The ultrasonic cleaning method can improve the cleaning effect of the cleaned object, and meanwhile, can prevent the surface and internal damage of the cleaned object.