The effects of different heat treatment regimes on the microstructure and properties of as-cast Mg-5Sn-1.5Al-1Zn-1Si alloy were investigated. The results indicate that during the dissolution of solid, as the dissolution time is prolongsed, the alloy dendrites gradually dissolve and the grains gradually spheroidize. It is shown that the appropriate dissolution parameter is 510 ℃ × 8 h, and the alloy after such treatment has an evenly distributed microstructure, with a trace of precipitation of fine second-phase particles from the matrix, and thus possesses a high elongation rate. The appropriate aging parameter for the alloy is 200 ℃ × 16 h, and due to precipitation and migration of the phases segregating at the grain boundary, the alloy after aging treatment possesses clear grain boundaries and a microstructure with high uniformity, and also has the yield strength up to 136.3 MPa, which is higher than that of the as-cast alloy by 16.5%. After a treatment including 8 hours of solid dissolution at 510 ℃ and 16 hours of aging at 200 ℃, the alloy has its uniformity and dispersion of the microstructure further improved, and its tensile strength and hardness reach to 178.2 MPa and 59.6HB, respectively, up by 21.6% and 23.4% compared to the as-cast alloy.