Abstract: 304 stainless steel was corroded in Al-12Si-xCu (x=0, 5, 10, 15, mass fraction/%) alloy melts at 620℃ for 120 h. The effect and mechanism of Cu content on corrosion behavior of the stainless steel were studied. The results show that the corrosion rate and the corrosion layer thickness of 304 stainless steel decreased with the increase of Cu content. The surface corrosion layer was composed of inner corrosion layer and outer corrosion layer. The microhardness of outer corrosion layer increased and that of inner corrosion layer decreased with the increase of Cu content. The corrosion reaction mainly occurred between Fe, Cr atoms in the stainless steel and Al, Si atoms in the alloy melt. Cu atom was not involved in the corrosion reaction. However, the addition of Cu reduced the diffusion activation energy of corrosion reaction, resulting in the decrease of corrosion ability of the alloy melt.