In this paper, Al-7%Si-0.3%Mg-x%Sc (X=0, 0.1, 0.2, 0.3, 0.5 and 0.8) casting alloys were prepared by vacuum induction melting furnace. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) were used to characterize the microstructure of the experimental alloy, and the refinement and modification mechanism of the experimental alloy was discussed. The results show that the main phases of the experimental alloy include α-Al, eutectic Si, Al₃Sc, AlSc₂Si₂ and iron-rich phases (β-AlFeSi and π-AlSiMgFe). The microstructure of the experimental alloy was refined by adding Sc element. With the increase of Sc content, the dendritic spacing of α-Al decreases and the size of eutectic Si decreases. When the content of Sc is 0.3%, the effect on refining of the experimental alloy is the best. A large number of fine Al₃Sc particles are produced in the alloy containing Sc. Al₃Sc particles are coherent with α-Al matrix, and the lattice mismatch between them is 1.0%. Therefore, Al₃Sc can be used as an effective heteronucleation particle of α-Al to fine α-Al After the addition of Sc, the eutectic Si will deteriorate and get refined. This is because the rare earth Sc elements existing in the eutectic Si can be adsorbed on the eutectic Si{111} dense surface as impurity elements, thus promoting the formation of high-density twins for the eutectic Si. In addition, AlSc₂Si₂ formed in the alloy can consume part of Si element, which reduces the amount and size of eutectic Si.