The microstructures, mechanical properties and eutectic silicon growth mechanism of A356 aluminum alloy treated by novel Al-5Ti-1B-1RE and Al-10Sr master alloy with individual or composite refinement and modification were studied. The results showed that under individual addition condition, the Al-5Ti-1B-1RE master alloy had a significant refinement effect on α-Al phase in A356 aluminum alloy and the tensile strength σb, yield strength σs and Vickers hardness of the alloy were significantly improved. Al-10Sr master alloy had a strong modification effect on eutectic silicon, and the elongation δ of the alloy was obviously improved. Under composite addition condition, the shape and size of α-Al phase in A356 aluminum alloy became more uniform and finer, and the grain boundaries were clearer. The eutectic silicon phases were almost converted into dispersed, fine fibrous shape, and the lamellar eutectic silicon almost completely disappeared. The length of the eutectic silicon was reduced from the as-cast state of 40-60 μm to 1-2 μm, achieving a complete modification effect. Its mechanical properties were much higher than those of the as-cast, any single refiner and modifier treated A356 aluminum alloy. The growth mode of eutectic Si in the unrefined and unmodified A356 aluminum alloy was typical small-plane step growth. The eutectic silicon with composite refinement and modification was mainly grown by the twin plane re-entrant edge mechanism, and the growth characteristics of the facet were gradually weakened until disappeared.