Abstract: In order to improve the structure and corrosion resistance of the magnesium alloy micro-arc oxidation coating,in the aluminate-phosphate composite electrolyte system containing Al₂O₃ powder particles, a composite ceramic layer was prepared on the surface of AZ31B magnesium alloy by using ultrasonic assisted micro-arc oxidation technology to study the effect of ultrasonic power ratio on the micro-arc oxidation ceramic layer. The microscopic morphology of the MAO film layer was observed by scanning electron microscopy（SEM）. The phase composition of the MAO film layer was analyzed by X-ray diffraction（XRD）, and the corrosion behavior of the MAO film layer in the electrolyte was measured by electrochemical polarization. The experimental results find that at the ultrasonic power ratio of 60%, the surface pores of the prepared micro-arc oxidation composite ceramic layer are less, the film thickness is the largest, and more MgAl₂O₄ phases appear,which shows the best corrosion resistance and the corrosion rate decreases to 1.05 mg·m^-2·h^-1.