High power single frequency fiber lasers are extensively applied in coherent detection and power spectrum beam combination etc. The suppressing method of stimulated Brillouin scattering was analyzed. The influences of amplification level on stimulated Brillouin scattering were studied. A single frequency fiber laser with linewidth of 70 kHz was used as seed source. And through two stage amplified, the central wavelength of 1 064.1 nm, linewidth of 70 kHz and power of 180 W were achieved respectively. The optical-optical efficiency is 71.1%. And the beam quality is M²_x=1.2 and M²_y=1.21. The output power before and after changing amplifier characteristic was compared. And the reason that output power increased was analyzed. The stimulated Brillouin scattering gain coefficient was reduced by varying amplifier temperature distribution. Then stimulated Brillouin scattering threshold was promoted. Finally, this makes the output laser power increase greatly after varying amplifier temperature distribution. The output power of this laser is only limited by pump power. If the pump power is increased, the higher power of single frequency all fiber laser will be achieved.