In this work, the method preparing micro-arc oxide coating on AZ31B magnesium alloy surface was studied, the formation process of AZ31B magnesium alloy micro-arc oxide ceramic layer was explored by the content of Na2WO4 in electrolyte, and the microstructure and corrosion resistance of AZ31B magnesium alloy were characterized. By using NaAlO2 as the basic electrolyte and adding a small amount of NH4VO3 and Na2WO4 on the surface of AZ31B magnesium alloy, a black micro-arc oxidation ceramic layer was prepared. Scanning electron microscope (SEM) and EDS were adopted to analyze the surface morphology and composition of the coatings with different contents of Na2WO4 and XRD was applied to analyze the phase structure. Electrochemical workstation was used to test the self-corrosion potential and self-corrosion current density of the micro-arc oxidation ceramic layer, and the CIE chromaticity of the coating layer was measured by 3nh chromaticity meter. After experimental verification, NH4VO3 with interaction to Na2WO4 contributed to the successful preparation of the black micro-arc oxidation ceramic layer. Through observation, it was found that with the increase of Na2WO4 content, the arc started to drop. The arc starting voltage was reduced from 320 V to 210 V. In the process of preparation, the micro-arc oxidation coating density was significantly increased and coating thickness increased firstly and then decreased. With the increase of Na2WO4 content, CIE chromaticity decreased firstly and then increased, and the corrosion resistance of the sample increased. When the content of Na2WO4 was 0.5 g/L, the micropores on the surface of the coating were evenly distributed, uniform in size and low in porosity, and the thickness of the coating was the largest at this time. The thickness was 23.8 μm. The CIE chromatic value was the smallest, which was 27.99, and the corrosion resistance was the best. The electrochemical test in 3.5% NaCl solution showed that the self-corrosion potential was ?0.138 V and the self-corrosion current density was 2.36×10–7 A/cm2, which decreased by 3 orders of magnitude compared with the matrix. At the same time, compared with the corrosion current density of 1.15×10–5A/cm2, its corrosion current density under the condition of no Na2WO4, decreased by 2 orders of magnitude. However, when sodium tungstate was added to 1 g/L, the coating appeared ablative phenomenon and the corrosion resistance decreased. The self-corrosion current density is not different from that of the matrix. It is concluded that the increase of Na2WO4 content will change the arc in the process of micro-arc oxidation and reduce the arc starting voltage. An appropriate increase of Na2WO4 will increase the thickness of the coating layer, reduce the CIE value of the coating layer, make the distribution of micropores on the surface of the ceramic coating layer more uniform and compact, improve the compactness of the coating layer, and thus improve the corrosion resistance of the coating layer. However, when the content of Na2WO4 is too high, the concentration of ions in the coating layer will increase, and more ions will adsorb to the surface of the coating layer for reaction. The resistance of the coating layer increases, leading to the rise of dielectric breakdown voltage, resulting in the surface ablation of the coating layer and decrease in coating densification and corrosion resistance.