The surface of MB8 wrought magnesium alloy was treated by HJ-III type ultrasonic impact treating (UIT) machine. The microstructures on the treated surface were analyzed by means of an optical microscope, and the grain refining mechanism of UIT was investigated by High Resolution Transmission Electron Microscopy (HRTEM). The wear resistance of MB8 wrought magnesium alloy was experimentally researched both with the treated and un-treated specimens respectively. The effect of UIT process on the microstructure, micro-hardness were also investigated using scanning electron microscope (SEM) and micro-hardness tester. The experimental results indicated that the grains on the top surfaces of MB8 were highly refined. The severe plastic deformation was formed by UIT, and the thickness of the plastic deformation layer was approximately 180 μm. The microhardness and wear resistance of the treated surface layer of MB8 could be enhanced significantly compared to that of the un-treated specimens. The longer the UIT time, the greater of the microhardness and wear resistance of the treated surface layer of MB8 would be. When the impact current is 1.2A and impact time is 6 min respectively, the microhardness and wear resistance of the treated sample is about 102.8% and 51.83% higher than that of the un-treated specimen, respectively.