Abstract: The phase constitution, tensile properties and failure mechanism during tensile process of as-cast, as-solutioned and as-aged samples of a Mg-6Al-6Zn-3Sn-1Y-0.5Mn (AZTYM66310) magnesium alloy are investigated. The α-Mg primaries in the microstructure of as-cast AZTYM66310 alloy appear typical equiaxed dendrites morphology, and a large number of interdendritic phases formed during solidification, including Mg + Mg32 (Al, Zn) 49 eutectic, Mg2Sn divorced eutectic phases, Al2Y phases and Al8Mn4Y phases. After the solution treatment of 380℃ × 6h, the majority of Mg32(Al, Zn)49 phases and part of Mg2Sn phases were dissolved into the matrix, contributing the contents of Al, Zn and Sn in the matrix increasing significantly, while Al2Y phase and Al8Mn4Y phase not changing significantly. Solution treatment significantly improves the tensile strength and the elongation of the alloy. After aging treatment of 380 ℃ × 6h + 150 ℃ × 16h, a large number of nano-scaled particle reinforced phases precipitated in the alloy, especially near the grain boundary, contributing to the yield strength increasing to 187.4 MPa. During the tensile process of the alloy, the second phases in the structure first break or separate from the matrix, resulting in the generation of microcracks. And then these microcracks propagate along the grain boundaries, eventually causing the sample to fracture.