An all polarization maintaining mode-locked fiber laser which is based on bias phase shift nonlinear loop mirror is proposed. An all polarization maintaining mode-locked oscillator system is set up. The system generates pulses with average power of 2.4 W, repetition rate of 20.3 MHz and pulse width of 142 fs by three stages Yb-doped fiber amplification. Since the oscillator adopts all polarization maintaining fiber structure, the mode locking is environmentally stable.