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To improve the corrosion resistance of Mg alloys, an excellent anti-corrosive composite coating containing iron oxide was prepared on AZ31B Mg alloy by dipping method. Micro-arc oxidation (MAO) treated Mg alloys were modified by phytic acid/3-aminopropyltrimethoxysilane hybrid and ferrous sulfate heptahydrate/stearic acid solution in sequence. The surface morphology feature, composition and structure, and functional groups were examined by SEM, EDX, XPS, and FT-IR, respectively. Corrosion resistance of the composite coating was evaluated by electrochemical tests in 3.5% NaCl solution. The results confirmed that the as-fabricated composite coating improves corrosion resistance by about two or four orders of magnitude, compared with the single MAO coating and Mg alloy substrate. The surface of the composite coating, which is containing iron oxide with a flower-like network structure in nanoscale, represents an excellent sealing effect for micro-defects of the MAO coating, effectively restricting the penetration of a corrosive medium into the Mg substrate.