The wavefront of the output high power laser beam generated by a MOPA laser system with slab gain medium was measured and analyzed by an H-S wavefront sensor, the wavefront measured consisted the static aberration of the whole optic chain and the dynamic component caused by the laser driver. Analyzing results showed that the temporal frequency of the beam aberration was mostly less than 10Hz, but the spatial distribution of the aberration was more challengeable to the phase compensation. Due to the narrow rectangle shape of the laser gain medium and the none uniformity of the pumping and cooling, it had some local big slope cross the beam section, that caused a lot of difficulties to the aberration measuring and wavefront correcting, there were many high order components in the wide direction cross the beam, but the phase aberration was more smooth in the vertical direction. A beam expander which has different ratio in the x/y directions had been used for matching the beam aperture and the valid controlling range of the deformable mirror. An adaptive optical system consisted of an H-S wavefront sensor and a deformable mirror (DM) with 67 valid actuators which arranged in two dimensions as hexagon designed to compensate the phase aberration. The driving vector of the deformable mirror was calculated from the spot array which sampled by the H-S wavefront sensor from the target beam, the surface response of all actuators were pre-calibrated and saved as a matrix. The matrix invert method was used to calculate the driving vectors in the close loop steps. The beam quality factor β was adapted to evaluate the output beam from the adaptive optical system. Simulation and experiment results of the close loop correction about the adaptive optical system showed that the aberration was compensated to a very low level and the far field beam quality of the high power laser could reach 1.67xDL.