To determine the amplitude of weak sinusoidal water surface acoustic wave (WSAW), a method based on the spectrum analysis of the phase-modulated interference signal is developed. Calculated from the amplitude spectrum of the detection signal, a characteristic ratio indicates that the phase-modulation depth of a WSAW is suggested by determining the amplitude of a WSAW according to their functional relationship. Experimental investigations for a 4 kHz WSAW evaluate the measurement’s precision with an amplitude measurement standard deviation of 0.12 nm. The measurement accuracy also is demonstrated by the experimental investigations. The theory of this method is briefly described, and the experimental setup is presented.