Magnesium alloys are considered to be potential biodegradable materials presently,the main actual limitation for the use of magnesium alloys is its too fast corrosion rate in the physiological environment.In order to improve the corrosion resistance of magnesium alloys,oxide-based ceramic coatings containing Ca and P were prepared on AZ91D Mg alloy by plasma electrolytic oxidation (PEO) technique in NaOH system.The phase composition,morphology and the element distribution of the coatings was studied by X-ray diffraction,scanning electron microscopy and energy dispersive spectroscopy.Electrochemical impedance spectroscopy (EIS) and Tafel polarizing curve were measured through CHI604 electrochemical analyzer in 0.9 % NaCl solution The results show that the coating was mainly composed of MgO,P and Ca joined the PEO process and only existed in the form of amorphous state in the coating,which influenced the surface morphologies of the coatings.The coating is of double-layer structure with the porous outer layer and the compact inner layer.The established “equivalent circuit” of EIS illustrates the changing regularity of the coating structure and corrosion resistance,which is consistent with the results of the Tafel polarizing curves test.Compared with the Mg alloy substrate,the corrosion resistance of the coating samples was improved by one or two orders of magnitude in 0.9% NaCl solution.