In high power fiber amplifier system, stimulated Brillouin scattering (SBS) becomes the primary limiting factor to achieve high power because of its relatively lower threshold. In order to improve SBS threshold, a new method to suppress SBS is presencted, which needs to modulate the incident seed laser both in phase and intensity. A model of SBS dynamics to numerically simulate the SBS threshold is established after using this method. The simulation results show that this method using phase modulation and intensity modulation simultaneously can greatly enhance SBS threshold under the condition of narrow linewidth output. When 20 W seed laser is put into a fiber amplifier system after phase modulation of 100 MHz, 10 V and intensity modulation of 10 MHz, 4 ns, the output power of fiber amplifier can reach 1127.4 W, which is 18 times of that of the SBS threshold for single frequency laser.