The effects of heat treatment on microstructure and mechanical properties of the as-extruded Mg-9Sn-1.5Y-0.4Zr (wt%) alloy were investigated experimentally in this study. The results showed that the heat treatment plays an important role on the microstructure and mechanical properties of the as-extruded Mg-9Sn-1.5Y-0.4Zr alloy. The as-extruded alloy is mainly composed of non-uniform Mg2Sn phase. After the solution treated at 495℃for 10h, a majority of the Mg2Sn phases in the alloy were dissolved into the matrix. Aging treatment can highly improve the mechanical properties of the Mg-9Sn-1.5Y-0.4Zr alloy with the optimized experimental condition of aging at 250℃for 60h after solution treated at 495℃for 10h. The corresponding mechanical property parameters are: hardness of 89HV, ultimate tensile strength of 262MPa, yield strength of 218MPa, elongation of 10.4%. Based on the experimental results analysis, it is found that the precipitation strengthening is the main contributor (~51.76%) to the total yield strength in the aged experimental alloy.