A 100 watt and ytterbium doped fiber laser based on MOPA structure is reported in this paper. The laser uses a modulated semiconductor laser as the seed source with pulse width of 10-350 ns and repetition frequency continuously adjustable in the range of 90 kHz-1100 kHz. Using 30/250μm large mode area fiber, the stable nanosecond pulse width is obtained by setting the optimum fiber length of 6 m. The second stage main amplifier uses 6 30W pumps, and the reverse-pump mode. When the repetition rate is 90 K Hz, the average power output of 100 W is obtained by the main power amplifier (MOPA). At the highest output power, due to gain shaping mechanism, the pulse width is reduced from 375 ns to 350 ns. The corresponding peak power is 10 kW, the single pulse energy is 1.11 mJ, and the output signal-to-noise ratio reaches 36 dB at the highest output power. And at the moment, the SRS effect happens just at the center wavelength 60NM. In this paper, a tunable 100-watt nanosecond fiber laser with two-stage amplification based on semiconductor modulation technology is presented. A compact 100W prototype has been completed.