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The microstructures and mechanical properties of as-cast Mg-5Sn-1Si magnesium alloy modified with trace elements Y, Bi, Sb and Sr were investigated and compared. Results show that the microstructure of the as-cast Mg-5Sn-1Si alloy consists of α-Mg, Mg2Si, Mg2Sn and Mg2(SixSn1-x) phases. After adding 0.8wt.％ Y, 0.3wt.％ Bi, 0.9wt.％ Sb and 0.9wt.％ Sr, respectively into the Mg-5Sn-1Si magnesium alloy, Mg24Y5, Mg3Bi2, Mg3Sb2 and Mg2Sr phases are precipitated accordingly. Trace elements can refine α-Mg grain and Chinese script-shaped Mg2Si phase. Refinement efficiency of different trace elements on α-Mg grain and Mg2Si phase is varied. Sr element has the best refinement effect, followed by Sb and Bi, while Y has the least refinement effect. Mg-5Sn-1Si-0.9Sr alloy has higher tensile properties than the other three modified alloys. The refinement mechanism of Y, Bi and Sr elements on Mg-5Sn-1Si magnesium alloy can be explained by the growth restriction factors and the solute undercooling. For Mg-5Sn-1Si-0.9Sb alloy, the heterogeneous nuclei of Mg3Sb2 phase is the main reason for the refinement of grains and second phases.