The embodiment of the invention provides a liquid flow measuring method, device and system, and belongs to the technical field of electronics. The method comprises the steps of continuously obtaining three reference liquid level depths, obtaining a liquid level depth contrast threshold on the basis of the three reference liquid level depths, obtaining a liquid level depth to be converted on the basis of the three reference liquid level depths and the liquid level depth contrast threshold, and then obtaining a liquid measuring mode corresponding to the liquid level depth to be converted on the basis of the conversion relation between the preset liquid level depth and liquid flow. The problem that due to large liquid surface fluctuations, the error of a liquid measuring value measured by an existing ultrasonic flowmeter is large is solved.