Phase modulation is an effective method to increase the threshold of the stimulated Brillouin scattering. Effects of modulation amplitude and frequency on the threshold are analyzed in theory. Experiments at different modulation amplitudes and frequencies are conducted to investigate the variance of the threshold. It is shown that the threshold increases with not only the modulation amplitude but also the modulation frequency. When the modulation amplitude is 7.7 and the modulation frequency is 25 MHz, the threshold is improved by about 7 dB. When the modulation amplitude is 1.5 and the modulation frequency is as small as 5 MHz, the effect of phase modulation on the threshold is not obvious.