The microstructure, composition and elemental depth profile of micro-arc oxidation film was analyzed by means of Scanning Electron Microscope (SEM), X-ray diffraction (XRD) and Energy Disperse Spectroscopy (EDS).The film is composed of an outer loose layer and an inner dense layer, a few circular pores present on the film surface, but merely in the outer loose layer, do not traverse through the inner dense layer to the metal substrate.The film is composed of forsterite phase Mg2SiO4 and magnesia MgO.The elemental depth profile shows that Mg, Si and O have a gradient distribution within the film.The gradient distribution of Mg, Si, O also indicated that the relative content of Mg2SiO4 decreased, while MgO increases in the depth direction to the alloy substrate.Combined with the impedance spectra collected after different exposure time to 3.5 % NaCl solution, a four-stage corrosion process was proposed to describe the corrosion behavior of micro-arc oxidation film on magnesium alloy.