Abstract: Semi-solid casting can improve the strength and toughness of Al-Si-Cu-Mg casting alloys by changing the microstructure of the alloy. Taking Al-7.0Si-xCu-0.5Mg（x =0, 1, 2） alloys as the object, the effects of Cu content on the microstructure and mechanical properties of the alloys were discussed, and the differences in microstructure and mechanical properties of the alloys under semi-solid casting and gravity casting were compared. The results show that the grains and the second phase at the grain boundary of the semi-solid casting alloys are refined, the dissolution and diffusion of Cu element promotes the precipitation of the nano-strengthened phase during aging process, and the strength of the alloys improves. The increase of Cu addition increases the number of nano-strengthened phases during aging and improves the yield strength and tensile strength of the alloy, but the elongation decreases slightly. For the alloys with the same Cu content, the semi-solid casting alloy has better yield strength, tensile strength and elongation than the gravity casting alloy due to fewer shrinkage holes and better structure density.