Aiming at the fluctuation problem of phase modulation depth of sinusoidal phase modulation interferometer, a phase modulation depth measurement method based on fast Fourier transform (FFT) and a proportional integral differential (PID) control scheme are proposed. A computational model for the measurement of phase modulation depth is established. The model performs an FFT of the original interference signal to calculate the amplitude attenuation ratio of the low-frequency component of the signal after the carrier frequency shift and solves the phase modulation depth according to the functional relationship. The incremental PID control combined with direct digital synthesis (DDS) is used to adjust the carrier signal strength in real time to realize the precise control of the phase modulation depth. Numerical simulation results show that the phase modulation depth measurement method can accurately realize the measurement function. Several sets of measurement experiments are carried out on solid surface micro vibrations excited by acoustic radiation using a sinusoidal phase modulation interferometer. The experimental results show that the interferometric signal demodulation results are better, and the signal-to-noise distortion ratio is better than 99.85% under the closed-loop control of the phase modulation depth. The results validate the effectiveness of the proposed method. The proposed scheme is of great application value for improving the measurement performance of laser sinusoidal phase modulation interferometers.