The aims is to study the anti-corrosion electrochemical responses of micro-arc oxidation (MAO) coatings with different thicknesses contrastively. AZ91D magnesium alloys were treated in silicate-containing electrolyte by adjusting the voltage in order to obtain MAO coatings with different thicknesses, the microstructure, element and phase composition of these coatings were studied by using SEM, EPMA and XRD, respectively. And the anti-corrosion electrochemical responses of the coatings were studied contrastively by cyclic voltammetry (CV), potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS). Anti-corrosion property of AZ91D magnesium alloys is significantly improved after MAO treatment, and the anti-corrosion electrochemical responses of the coatings with different thicknesses vary. Compared with the thin coating prepared under low voltage, the closed ring area surrounded by the CV curve and the corrosion current density of the thick coating prepared under high voltage both reduce about one order of magnitude, and its linear polarization resistance increases approximately 3.7 times. Meanwhile, its impedance modulus is higher and thus it exhibits more superior anti-corrosion performance. At the same time, the corrosion processes of two kinds of coatings are different, which can be divided into two stages for the thick coating, namely, the corrosive medium gradually penetrates into the coating, and then down to the interface between the coating and the substrate, eroding the substrate. While the deterioration of the thin coating mainly experiences three stages:the corrosive medium gradually infiltrates into the coating and penetrates through the coating/substrate interface, then corrodes the substrate, and finally the coating completely fails. In conclusion, the thick MAO coating has weak corrosion tendency and excellent anti-corrosion property, which is supported by the above three electrochemical testing results that reveal the corrosion process and mechanism of the thick coating with different degree respectively.