Magnesium alloy AZ91D was processed respectively in one, two, three and four-component electrolytes by using AC microarc oxidation technique. The corrosion resistance of AZ9 1D alloy was measured by electrochemical methods. The optimum electrolytes in two, three and four components were found. In four-component electrolyte composed by NaOH,NaAlO2, H2O2 and C4H4O6Na2, the film formed on AZ91D alloy is smooth and compact, and has a higher corrosion resistance. The effect of the ingredients in electrolytes was discussed based on their roles in the formation of corrosion resistant film.