The phase modulator is used to increase the threshold of the stimulated Brillouin scattering. Effects of modulation depth and frequency on the threshold are analyzed. The increase of the threshold is determined by the modulation depth which is related to the half-wave voltage. A heterodyne method to measure the half-wave voltage of the phase modulator is proposed in order to determine the modulation depth. The results of the experiments demonstrate that the half-wave voltage is 3.7 V when modulation frequency is 50 MHz. The variation of the stimulated Brillouin scattering threshold are researched through experiments when the modulation frequency is 50 MHz and modulation depth is 1.435 and 2.631. Results show that the threshold increases 6.7 dB when modulation depth is 2.631, which matches the theoretic calculation. High threshold is obtained with low power and low modulation frequency.