Based on the empirical electron theory of solids and molecules, the VESs of the matrix a, Mg-Ag segregation area, W phase and the interfaces of W/Mg-Ag/a, W//a were calculated after studying the interface atom structure between the matrix a and W phase, then the relationship between the interface VESs W/Mg-Ag/a and the interface properties was analyzed in Al-Cu-Mg-Ag alloy in this paper. The results show that the continuity of the outer of W/Mg-Ag/a is better than that of the inner because that the electron density difference of Mg-Ag/a is 16.54% and that of W/Mg-Ag is 50.73%. The Mg-Ag layer makes the interface electron density difference of W/a decrease by 1.13%, while it makes the strongest covalence band force and the covalence electron density in (111)a increase by 14.52% and 146.87% respectively, it also makes the strongest covalence band force and the covalence electron density in (111)W increase by 45.85% and 45.30% respectively. The existence of Mg-Ag layer enhances the alloy strength and toughness owing to it increasing the obstruction of W phase to the dislocation, the interface continuity, the interface binding force and stability while it decreasing the interface electron density differences and the interface stress.