In order to meet the needs of heat-resistant magnesium alloy applications, Mg-5 Si base alloys were used as a research object and Mg-5 Si-Y alloys were prepared by permanent mold casting method. The microstructure and composition of the alloys were analyzed by scanning electron microscopy, energy dispersive spectroscopy, optical microscopy and ICP spectrometry. The yield strength, tensile strength, elongation and hardness of the alloys were tested on cupping machine and Vickers hardness tester. The effect of Y content on the microstructure and mechanical properties of Mg-5 Si alloy was discussed. The results show that the morphology of primary Mg2Si phase changed from dendritic shape to polygonal shape, and that of eutectic Mg2Si phase changed from Chinese script shape to lamellar shape with the addition of various amount of Y. When the mass fraction of Y was 0.4% or 0.8%, α-Mg dendrites were refined. When the mass fraction of Y was 0.8%, the mechanical properties were the best. The tensile strength was 107 MPa, the yield strength was 91 MPa, the elongation was 5%, and the hardness was 138 HV30. Meanwhile, some white bulk Mg-Y-Si compounds were found in alloys with the Y mass fraction 1.2%.