A method and device for on-line foam quality acoustic monitoring are suggested. As per the invention, the method provides for the following: at least one emitter / receiver pair is placed in a close vicinity to the foam flow; at least one acoustic pulse is emitted; time, within which the acoustic pulse overcomes the way from the emitter to the receiver, is recorded; acoustic pulse speed (sound speed) is determined by analyzing an acoustic response of the receiver. After that, pressure in the foam in the area between the emitter and receiver is defined and the foam quality Γ is calculated as per the equation:Γ=12±14-NρflpCfm2,where: Cfm—speed of sound in the foam, p—pressure, ρfl—fluid density, Γ—foam quality, N—polytrophic expansion coefficient (N=1 for isothermal process; N=1.4 for adiabatic process), in case of a foam composed of a perfect gas and a perfect fluid for cases, when the foam quality is supposedly much lower than ½ or much bigger than ½, the sign to be selected in the said formula is <<−>> and <<+>>, respectively; in cases when both values of Γ are near ½, the both quality values should be considered as a possible option, or in more complicated cases, it can be found from the Value Spreadsheet