A magnesium alloy with excellent ignition-proof property was acquired by adding yttrium and calcium at the same time. The Mg-3.SY-0.8Ca alloy could be immune from ignition at 1173 K for 30 min. XRD, SEM and AES analysis of oxide scale revealed that the oxide film formed on the surface of Mg-3.SY-0.SCa alloy was mainly composed of Y2O3. The Y2O3 film may be considered as an outstanding protective film due to its stable crystal structure, high melting point, low steam pressure, the P-B ratio more than 1, compact structure, low conductivity and excellent ductibility and so on.