The oxidation mechanism for the molten surface of Al-5Mg-2Si-Mn alloy was studied. The results show that the oxide layer contains MgO, Al2O3, MgAl2O4, BeO and SiO2, and it is composed of a composite inner layer (MgO/Al2O3/MgAl2O4/BeO/SiO2) and an outer layer of MgO. An oxidation mechanism was proposed to describe the four oxidation stages which included oxidation adsorption, accelerated oxidation, transitional oxidation and stable oxidation. The effects of oxidation time and oxidation temperature on the thickness of oxide layer were discussed. Thermodynamic calculations were used to confirm the feasibility of oxidation process, indicating that MgO was the most stable oxide in the experimental temperature range. Further, the stable regions of MgO and MgAl2O4 as functions of magnesium content and oxidation temperature were calculated. In the stable oxidation stage, the diffusion activation energy of Mg atoms in MgO was fitted according to the outer layer thicknesses, and the kinetic equation of the outer layer thickness with the oxidation time and oxidation temperature was established. From the perspective of thermodynamics and kinetics, the oxidation products of the alloy during the melting process and the effects of oxidation time and temperature on the oxide layer were analyzed. [GRAPHICS] .