Phase modulation of the single frequency laser to attain the multi-tone laser costs less and is easy to carry out. It has been proved to be an effective way to suppress stimulated Brillouin scattering (SBS) in narrow-linewidth fiber amplifiers. Relationships between modulation signals and constitutions of the modulated laser are studied theoretically. When the modulation frequency is small, for example 100 MHz, the multi-frequency laser can be attained and SBS threshold is effectively enhanced. Experimental researches are done to study effects of 100 MHz phase modulation on SBS mitigation of high power narrow linewidth fiber amplifiers. SBS threshold output powers are 2.5 times and 3.7 times of that of the corresponding unmodulated amplifiers with 16 and 26.5 m delivery fiber respectively. Experimental results show that SBS in narrow-linewidth fiber amplifiers can be effectively suppressed with phase modulation and the suppression effect depends on amplifiers′ parameters. The lower the SBS threshold without modulation, the better the suppression effect.