Abstract: The theoretical formula of phase-shift coefficient of piezoelectric fiber optic phase modulator is introduced, and a measuring method of the phase-shift coefficient is suggested based on Youngs two-slit interference principle. The feasibility of this measuring method is analyzed, and then verified by experiments. A phase modulator device based on piezoelectric ceramic ring is developed, and the phase-shift coefficient is tested which is consistent with the theoretical result. A experiment of phase locking of two beams is carried out with the self-developed piezoelectric fiber optic phase modulator, and coherent beam combination in far-field is achieved successfully. The fringe visibility in phase-locking state is up to 43%, compared with 3% with phase-locking off.