The work aims to study the effect of ultrasonic pulse energy on the film formation process, microstructure, morphology, and corrosion resistance of micro-arc oxidation (MAO) coatings on AZ31B magnesium alloy, to further analyze the film formation mechanism. By varying the ultrasonic pulse frequency (50, 100, 150 kHz) to adjust the pulse energy, micro-arc oxidation was performed on the surface of the AZ31B magnesium alloy substrate in a silicate system. The surface morphology, elemental composition and phase structure of the coating were analyzed by laser confocal microscopy, SEM/EDS and XRD. The corrosion resistance of the coating was characterized through electrochemical testing. The phase composition of the micro-arc oxidation coating prepared in the silicate system mainly consisted of MgO and Mg2SiO4. As the ultrasonic pulse frequency increased, the pulse energy decreased, leading to an extended arc initiation time during the MAO process, a reduction of the micropores diameter, and an improvement in the uniformity of the coating surface. Compared with the substrate, the MAO coating prepared under ultrasonic pulse conditions exhibited an increase in self-corrosion potential by 1.5 V and a reduction in self-corrosion current density by 3 to 4 orders of magnitude. In conclusion, ultrasonic pulses voltage significantly affects the microstructure of magnesium alloy MAO coatings and enhances the uniformity of the coatings. With the increase of pulse frequency, which corresponds to a decrease of pulse energy, the size of the arcs during the MAO process reduces, and the number of arcs increases. Consequently, the size and diameter of the micropores on the surface of the MAO coating are reduced. At the same time, it also reduces the thickness of the loose layer and increases the thickness of the dense layer, which is great beneficial to improve the corrosion resistance of the coatings.