Abstract: Effects of strontium modification, boron modification, and compound modification of strontium and boron on microstructure and mechanical properties of as-cast and T6 state Al-6Si-3Cu-0.3Mg alloys were studied. Effects of strontium content (0.025wt%,0.050wt%,0.075wt%) on microstructure and mechanical properties of the alloys after compound modification were also studied. The results show that strontium modification, boron modification, and compound modification of strontium and boron all could refine the structure and improve the tensile strength of as-cast and T6 state alloys, and the effect of compound modification was the most remarkable. The tensile strength and elongation of as-cast alloy increased by 3.71% and 13%, respectively, compared with that of the modified alloy, and the tensile strength of T6 state alloy increased by 33.6%, but the elongation decreased by 41.9%. Under the compound modification condition, the primary α-Al was coarsened in the as-cast alloy with increasing strontium content, and the overall tensile strength did not change much. The average length of eutectic silicon in T6 state alloy decreased first and then increased with the increase of strontium content, and the tensile strength gradually increased. When the strontium content was 0.025wt%, the eutectic silicon size was the smallest, and the compound modified alloy showed the best plasticity.