In order to investigate the combing effect of Er and Sr on the microstructure and mechanical properties of A356 alloy, Al-10Er-5Sr master alloy was fabricated and its microstructure and phase compositions were investigated. The optimal percentage of Al-10Er-5Sr in A356 alloy was obtained according to the evolution of microstructure and mechanical properties. The results showed that α-Al, Al4Sr and Al3Er were the main phases in Al-10Er-5Sr. In addition, the aluminum alloy with 0.6 wt% Al-10Er-5Sr exhibited optimal microstructure and mechanical properties, with the secondary dendrite arm spacing (SDAS) decreasing to 20.2 μm and acicular-like eutectic silicon transforming to fibrous. Due to the improved microstructure, the ultimate tensile strength of the alloy (with 0.6wt% Al-10Er-5Sr) increased to 203.5 MPa, which are much better than that of untreated A356 aluminum alloy. Finally, the grain refinement and modification mechanisms were discussed.