Be addition make the oxide film formed on the molten magnesium alloy denser and tougher, thus the burning of the Mg during melting was suppressed. AES was employed to study the composition of the oxide film growing on the surface of the molten Mg-9Al-0.5Zn-0.3Be alloy. It was suggested that the surface oxide film could be divided to two layers: the outer MgO layer and the inner mixed layer consisting of MgO and BeO. Both experimental results and theory analysis indicated that the outer layer grew under parabolic law and the thickness almost kept unchanged during the steady growing state for the inner layer.