Experimental study on all-fiber polarization maintaining master oscillator power amplifier fiber laser is performed. A 5 m small core diameter double clad polarization-maintaining Yb-doped fiber is employed in the experiment by means of two-level cascade amplification. A more than 67 W polarization maintaining laser is obtained under a maximal 88 W pump power. In this case, the laser density in the fiber core is about 3.4 W/m2, and the optical-to-optical efficiency is 76%. A higher power laser output can be realized as the pump power is further enhanced.