Abstract: Mg-3Al-0.5SiO₂-xSn (x=0,1,3,5,7,10, mass fraction/%) alloys were melted and casted by taking pure magnesium ingot, pure aluminum ingot, pure tin ingot and nano-SiO₂ powder as raw materials. The effects of Sn addition amount on the microstructure and mechanical properties of the alloys were studied. The results show that with increasing Sn addition amount, the coarse primary Mg₂Si phase gradually transformed into granular or acicular phase, and the Mg₂Sn phase content increased. When the mass fraction of Sn was 5%, Mg₂Sn phase had the most uniform distribution and the smallest size. The strength and precentage elongation after fracture of the alloy increased and then decreased with the increase of Sn addition amount, and the fracture mode changed from brittle fracture to ductile fracture and then to brittle fracture. When the Sn mass fraction was 5%, the alloy had the best tensile properties at room temperature with the tensile strength, yield strength and precentage elongation after fracture of 145.6 MPa, 98.7 MPa and 7.12%, respectively, and the fracture mode was brittle fracture.