A phase analog coherent demodulation system includes a difference frequency and pulse generator configured to generate a pulsed light and a continuous light with different frequency from the pulsed light, a controller, and an I/Q demodulation module. The controller locks the frequency difference signal of the pulse light, and the pulsed light generates the backward scattered light of constant frequency in the measured medium, and the backward scattered light returns along the original path and mixing beat with the continuous light in the I/Q demodulation module, the difference frequency electrical signals of the two optical signals are obtained. The I/Q demodulation module orthogonally demodulates the difference frequency electrical signal by using the frequency difference signal of the pulsed light and the continuous light as the fundamental frequency signal of the orthogonal demodulation, to detect phase information of the vibration signal. The invention reduces the operating frequency required for quadrature demodulation in system phase detection.