The effects of Sr and Y addition on the microstructure and mechanical properties of A356 aluminum alloy were studied by metallographic microscope, scanning electron microscope, transmission electron microscope, energy dispersive spectra and mechanical property tests. The results show that the combined addition of Sr and Y can obtain the best microstructure and mechanical properties compared with the separate addition of Sr or Y. The eutectic Si changes completely from flake-like or acicular-like to fibrous shape, the equivalent grain diameter of α-Al decreases from 42.49 μm to 26.63 μm, the secondary dendrite arms spacing (SDAS) decreases from 22.38 μm to 13.60 μm, the tensile strength increases from 166.1 MPa to 185.3 MPa, the elongation increases from 3.6% to 7.6%, and the fracture mode changes from brittle fracture to ductile-brittle fracture. The refinement mechanism and modification mechanism of Sr and Y were discussed. It is concluded that the main reason for α-Al grain refinement is the rapid heterogeneous nucleation of α-Al based on Al₃Y, and the modification mechanism is impurity-induced twins which grow in the mechanism of twin plane reentrant edge.