The development and application of Mg alloys propose higher requirements on the oxidation resistance at high temperature, containing oxidation in air and in flame. Authors previous studied the oxidation resistance of Mg-Ca alloy in air and in flame respectively, and different oxidation behavior was found. In this paper, the difference of oxidation behavior and mechanism in different oxidative environments was discussed. Results show that Mg2Ca behaves different, which is unfavorable for the oxidation in air but beneficial for the oxidation in flame. For the oxidation in air, the Mg-Ca-O oxide film with Ca accumulation layer in Mg-Ca alloy with high Ca content shows good protective effect, and the low Mg vapor pressure exerts small force on the oxide film. However, the falling off of the oxide film on Mg2Ca results in the further oxidation. Hence, the Mg-Ca alloy with high Ca content show sound protective effect. For the oxidation in flame, the molten alloys free the Ca atoms to diffuse outward for the formation of Mg-Ca-O oxide film with accumulation layer of high Ca content in Mg-Ca alloy with high Ca content, which shows super protective effect. In spite of the high Mg vapor in the molten alloy, the Mg-Ca-O oxide film with high Ca accumulation layer show excellent protective effect.