A Yb-doped mode-locked fiber laser with a tunable pulse width was built by using a cladding pumped all-PM(Polarization Maintaining) “figure-of-eight” cavity. The Dissipative Soliton Resonance(DSR) square pulses with a central wavelength of 1 064 nm, a repetition rate of 4.683 MHz, a tunable pulse width range from 0.8 ns to 5 ns and an average power range from 74.8 mW to 429.7 mW, are achieved. There is no polarization controller in the cavity. The mode locking state can be self-started only by adjusting the pump input, and then can be keep stable for a long time, and without lock-lose even under the condition of violent vibration. By using the two stage fiber amplifiers to amplify the power, the laser output with an average power of 100 W and the peak power as high as 26.68 kW is achieved.