In order to suppress the stimulated Brillouin scattering effect and increase the amplification power of single frequency narrow linewidth seed source, master oscillator power amplifier (MOPA) structure was adopted. Fiber length, core diameter and pumping parameters were optimized. Optical output of 42W signal at 1064nm was realized. In the experiment, the first stage amplification used 914nm semiconductor laser as pumping source with gain fiber core diameter of 10μm and length of 8m. The second stage amplifier used 976nm semiconductor laser as the pumping source with gain fiber core diameter of 20μm and length of 2.4m. When seed power is 40mW, pump power of the first stage amplification is 6.8W and pump power of the second stage amplification is 85W, optical output 42W of the signal at 1064nm was obtained. The results show that light-to-light conversion efficiency is about 49.4%, polarization extinction ratio is 27.5dB; central wavelength of output signal light is 1064.5nm, and linewidth is about 70MHz. Single-frequency characteristics of seed light are maintained. Stimulated Brillouin scattering is not observed at 42W continuous output. Continuous increase of pump power is expected to achieve higher power amplification.