An all fiber linearly polarized narrow-linewidth nanosecond pulsed fiber laser based on master oscillator power amplifier (MOPA) structure was reported. The pulsed seed was generated by modulating a single-frequency distributed feedback (DFB) fiber laser using an electro-optic intensity modulator. In order to suppress stimulated Brillouin scattering (SBS), the pulse width was set to be 3 ns and the linewidth of seed was broadened to be 2.9 GHz by a phase modulator. After amplifying by two stages polarization maintaining Yb³⁺ doped fiber amplifier chain, the output pulse laser was obtained with an average power of 142 W, a repetition rate of 1 MHz, a pulse duration of 2.88 ns, corresponding to peak power of 49.3 kW. At maximum output power, the laser beam quality M² was ～ 1.15 and the polarization extinction ratio(PER) was >15.4 dB.