An optical fiber acousto-optic phase modulator coated with piezoelectric ZnO is discussed. The geometry of optical fiber using ZnO coating presented and the basic theory of piezoelectric resonator of this geometry is analyzed. The equivalent network model for the optical fiber acousto-optic phase modulator coated with piezoelectric ZnO is given. And then the reflection coefficient is obtained by this model. Computer simulation is made according to the practical parameters.