The photon doppler velocimetry system has the advantages of strong anti-interference ability and wide range of velocity measurement. It is suitable for low signal-to-noise ratio and poor signal quality measurement occasions. The working principle of photon doppler velocimetry system is introduced. Four commonly used signal demodulation methods, fringe method, phase demodulation method, short-time Fourier transform method and wavelet transform method, are described in detail. The four signal demodulation methods mentioned above are used to demodulate the doppler signal modulated by harmonic oscillation. The advantages, disadvantages and applicability of different signal demodulation methods are shown intuitively. The experimental results show that the phase demodulation method is most suitable for demodulating the vibration signal of photon doppler velocimetry system. Finally, the feasibility of using recursive Hilbert transform to reduce the orthogonality error of phase demodulation method is discussed, and the effectiveness of this method is verified by simulation experiments.