The effect of cerium on ignition temperature of AZ91D magnesium alloy was studied. By the addition of cerium of 1%, the ignition temperature is raised by 180 ℃, so the magnesium alloy added with cerium can be melted in air. The burning temperature increases with the increasing of cerium. The structure and chemical compositions of the surface oxide film were investigated by XRD and Auger electron spectrometry(AES). The results of XRD indicate that the oxide film of the surface of ignition-inhibition magnesium alloy can change from loose structure of simple magnesia to compact composite structure consisting of magnesia, cerium oxide, Mg17 A112 and aluminum oxide, which has excellent ignition-inhibition effect. AES depth profile analysis shows that the oxide film can be divided into three layers. The outside layer is mainly made up of magnesia, the middle layer, which consists of cerium oxide, magnesia, and aluminum oxide, is compound and compact. Thermodynamic analysis indicates that the structure of the surface oxide film is accordant to the change of free energy and high vapor pressure of magnesium.