We develop and demonstrate a method of optical phase modulation in the Pound-Drever-Hall (PDH) technique. The phase modulation in this paper is realized by an acousto-optic modulator (AOM) operating in the Bragg diffraction regime. In this process, a light beam separated from a laser (780 nm) is sent through the AOM twice and coupled to a high finesse Fabry-Perot cavity. Then, the light power coupling into the cavity is stabilized by modulating the optical amplitude with this AOM. The coupling light power is stabilized to a level of 10-3. In the meantime, the PDH error signal is obtained by modulating the optical phase with the same AOM. After the error signal is fed back to the laser current, the laser linewidth is suppressed to approximately 907.91 Hz. This method of phase modulation is simple and convenient, and we believe it can be widely used in modulation transfer spectroscopy and frequency-modulation sideband spectroscopy.