The oxidation of α-Mg and Mg₂Ca in Mg-Ca alloy was studied by calculating the adsorption process of O₂ on α-Mg (0001) and Mg₂Ca (0001) based on density functional theory (DFT), and the adsorption process and oxidation mechanism were investigated. Results show that during the adsorption, O₂ has a strong interaction with α-Mg and Mg₂Ca. The interactions are chemisorption due to the excellent E_ad values, but the adsorption structures of Mg₂Ca are not as stable as that of α-Mg. During the oxidation, O₂ reacts with Ca and Mg atoms in α-Mg and Mg₂Ca to form Mg-Ca-O oxide film, hence improving the oxidation resistance of Mg-Ca alloy. Since the adsorption structure of Mg₂Ca is not as stable as that of α-Mg, the oxide film formed by Mg₂Ca shows weaker protective effect on the substrate than that formed by α-Mg.