High energy nanosecond pulsed ytterbium-doped fiber laser is developed. The laser utilizes active Q-switching technology to generate nanosecond pulses, and the pulses can obtain high energy and average power after it passes through two-stage master oscillator power amplifier (MOPA) system. When the pulse repetition frequency is 100 kHz, the maximum average power of laser pulses is 302 W, pulse duration is 203 ns, single pulse energy is 3 mJ and the optical-to-optical conversion efficiency is 84%. Laser rust removal experiment is carried out using the laser, the experimental results show that the laser is stability, and the laser can meet the laser rust removal, laser paint removel and many other applications.