To obtain the high power dual-frequency laser source for lidar-radar system, a narrow-linewidth Nd∶YAG monolithic nonplanar ring-oscillator laser with central wavelength of 1 064 nm was uesd as a single-frequency seed laser. The output was split into two parts, one was transmitted in optical fiber and the other was frequency shifted by an acoustooptic modulator. The combined beam had power of 20mW and contained two frequency components with frequency separation of 150 MHz. A three-stage master oscillator power amplifier using diode laser pumped and Yb3+-doped quartz fiber as gain medium was proposed to boom the output power of dual-frequency laser. The maximum amplified power was 50.3 W, beam quality factor was 1.30 and the slope efficiency of the third-stage main amplifier was 74%. The amplitude ratio and frequency separation of the dual-frequency components were maintained in the amplification process. The modulation depth and signal-noise ratio of the beat signal had not deteriorated. Dual-frequency laser fiber power amplifier provides robust performance of both the stable beat frequency and the relative high output power.