In order to suppress the stimulated Brillouin scattering effect in narrow linewidth pulse fiber amplifiers, the linewidth of single-frequency seed source was expanded by using multi-spectral line technology. The experimental verification of high peak power pulse all-fiber laser based on three-line master oscillator power amplifier was carried out. The results show that, after two-stage preamplifier and one-stage power amplifier, the maximum average power of laser output is 303W, pulse width is 2.8ns, repetition rate is 3.1MHz, and the corresponding peak power is 35kW. At the highest power output, the beam quality of the laser is less than 1.3. The structure of three spectral lines has obvious inhibition effect on stimulated Brillouin scattering. This study provides a reference for the amplification technology of high peak power pulsed fiber lasers.