The invention relates to a compound additive for magnesium alloy micro-arc oxidation, which consists of trivalent rare earth compound, silane coupling agent and multi-component carboxylate, wherein the rare earth compound is one, two or three of cerium nitrate, neodymium nitrate and lanthanum nitrate; the amount of the rare earth compound is 0.1 to 1 percent; the multi-component carboxylate is one or two of tartrate, citrate, phytate, lactate and oxalate; the amount of the multi-component carboxylate is 5 to 20 percent; a coupling-agent additive is silane with an organic functional group; andthe amount of the coupling-agent additive is 1 to 8 percent. The compound additive can be used for various alkaline systems of magnesium alloy micro-arc oxidation of silicate, phosphate, aluminate and the like, and can perform magnesium alloy micro-arc oxidation at room temperature after full mixing. Under the same oxidation conditions, micro-arc oxidation film prepared by use of the additive canbe soaked in 3.5 percent NaCl solution for 400 hours with no corrosion, and the hardness of the film remains unchanged.