Micro-arc oxidation coatings were prepared on AZ91D magnesium alloy by changing the collocation of Na₂SiO₃, NaOH, KF and NaAlO₂ based on simplex-centroid mixture design. The effects of electrolyte components on the formability and corrosion resistance of the coating were investigated. The results show that the obtained regression equation is very significant and has high prediction accuracy. Pareto analysis shows that the four electrolytes have significant effects on the corrosion resistance of the coating. Response surface analysis shows that increasing the concentration of Na₂SiO₃ or NaAlO₂ can significantly improve the corrosion resistance of the coating. But the combination of the two is detrimental to the improvement of the corrosion resistance. The main salt is very important for increasing the coating formability and corrosion resistance. When the electrolyte does not contain the main salt, the coating formability and corrosion resistance are very poor. The appropriate increase of NaOH and KF concentration in the electrolyte containing the main salt is also beneficial to improve the corrosion resistance. According to Pearson correlation analysis, the corrosion resistance primarily depends on the coating density and porosity, and is also affected by other characteristic parameters such as thickness and composition. The components of the electrolyte affect the properties of the coating through influencing the above microstructure parameters.