We demonstrate a high-power single-frequency master oscillator power amplifier (MOPA) fiber laser. The central wavelength of the single-frequency fiber laser seed is 1 063.8 nm, with a linewidth narrower than 20 kHz and output power of 120 mW. By using two-stage amplification, a single-frequency fiber laser with an output power of 122 W is obtained, and the optical-optical conversion efficiency is 72%. No significant amplified spontaneous emission (ASE) or stimulated Brillouin scattering (SBS) is observed. The output power can be further increased by launching more pump power.