Abstract: The phase ambiguity and unwrapping of phase-sensitive optical time domain reflectometer was investigated. The whole process of phase change introduced by the perturbation was deduced, and the reason for phase ambiguity in the phase demodulation was analyzed. The piezoelectric ceramic was used as the disturbance source, and phase demodulation was performed by digital coherent demodulation. The experimental results show that phase ambiguity exists on each phase demodulation curve and among these curves simultaneously. So it is necessary to perform two phases unwrapping to eliminate phase ambiguity. Meanwhile, the phase disorder in the experiment was analyzed. It was pointed out that the phase unwrapping threshold and the phase drastic change in the disturbance position caused the inaccurate phase unwrapping of the perturbation position. The phase change in the adjacent position following the phase difference peak was used to restore the perturbation. The experimental results show that this method can correctly restore the disturbance signal, can demodulate the disturbance signal in the range of 10 Hz-1.5 kHz accurately, can simultaneously demodulate and respond to multiple perturbations along the optical fiber and the phase change amplitude have a good linear relationship to the disturbances intensity.