Experiments are conducted based on phase modulation in order to research the suppression on stimulated Brillonin scattering (SBS) effect at different modulation frequencies and voltages. A 0.17 nm Stokes frequency shift without phase modulation is observed, and the corresponding SBS threshold is about 120 mW. The phase modulation is realized by an electro-optic modulator whose half-wave voltage is 3.5 V. When the modulation voltage is 4 V and the frequencies are 1 MHz and 5 MHz, respectively, and the SBS threshold power is 250 mW and 440 mW. When the modulation frequency increases, the threshold power continuously increases. When the modulation frequency is higher than 30 MHz, the SBS effect is not observed below the damage limited power.