To achieve high-power, multi-parameter tunable nanosecond pulsed laser output based on an all-fiber master oscillator power amplifier(MOPA)structure, a semiconductor laser directly modulated by an electrical circuit is employed as the seed source. A high-power pulsed laser with an all-fiber MOPA structure is designed, optimizing the bending radius of the active fiber and the optical path configuration. The experimental results show that when the primary and secondary pump powers are 2.2 W and 34.38 W, respectively, the laser achieves stable output with an average power of 20.85 W, a maximum peak power of 12.9 kW, a central wavelength of 1 064.5 nm, a linewidth of 3.75 nm, and an average beam quality factor M² of 1.345.