The effects of Bi content on the microstructure, phase structure and composition, mechanical properties and fracture morphology of as-cast Al-5Mg-2Si-0.6Mn alloy were studied. The results show that when Bi content is 0.3%, it has the most obvious deterioration effect. Bi enrichment generates component supercooling at the front end of eutectic Mg2 Si growth interface, which reduces the actual supercooling degree of the liquid phase and limits the growth speed of eutectic Mg2 Si. Eutectic Mg2 Si phase changes from coarse fiber to fine point fiber before metamorphism. The mechanical properties of the alloy are significantly improved. The tensile strength and elongation of the alloy are increased from 205 MPa and 2.8% to 240 MPa and 5.3%, respectively, increasing by 17.1% and 89.3%.