Micro-arc oxidation coatings were prepared on magnesium alloy AZ91D substrates at the different applied voltages in aluminate solution. The morphologies, phase components, and corrosion resistances of the coatings were investigated. The effect of the applied voltages on the microstructure and corrosion resistance of the coatings was also discussed. The results indicate that the coatings are uniform in thickness, and mainly composed of MgAl2O4 and MgO. There were many residual discharging channels on the coatings surface. The coatings improved the corrosion resistances of magnesium alloy AZ91D considerably. With the increase of the applied voltage, the thickness and the MgAl2O4 content of the coatings increase, while no significant variation is observed in the MgO content. The coatings surface becomes lower porosity and larger pore size with increasing of the applied voltage. In addition, the corrosion resistances of the coatings on magnesium alloy AZ91D surface are obviously superior to the magnesium alloy AZ91D substrate in the 3.5 wt. % NaCl solutions, and the effect is more remarkable at higher voltage.