Abstract: Taking Al-5.5Mg-2.5Si-0.6Mn non-heat-treated cast alloy as research object, the effects of single modification with Sb (0.20wt%) and composite modification with Sb (0.20wt%) and Sr (0.03wt%, 0.05wt%, 0.10wt%) on the microstructure and tensile properties of Al-Mg-Si alloy were studied. The results show that after the single modification of Sb, the primary α-Al phase in the test alloy changed from equiaxed shape before modification to dendritic morphology, with an increase in average size, and the Al-Mg₂Si eutectic structure still maintained a fine fibrous shape. After the composite modification of Sb and Sr, the dendritic size of the α-Al phase increased, and the eutectic structure changed from fibrous shape to labyrinthine shape. When the mass fraction of Sr reached 0.05%, the eutectic structure was completely modified, and as the Sr mass fraction increased from 0.03% to 0.10%, the size of the α-Al phase and the interlayer spacing of the eutectic structure first increased and then decreased. Compared with those of the unmodified alloy, the tensile strength and percentage elongation after fracture of the test alloy after the single modification of Sb increased by 6.0% and 63.3%, respectively; there were numerous small ductile dimples in the tensile fracture, exhibiting typical ductile fracture characteristics, and the alloy had the best tensile properties. After the composite modification of Sb and Sr, the tensile properties showed a significant decrease, and with the increase of Sr content, the tensile strength and percentage elongation after fracture first increased and then decreased; the tensile fracture was composed of cleavage planes, shrinkage cavities and cracks. The single modification effect of Sb was superior to the composite modification effect of Sb and Sr.