The effect of Sc content on microstructure, mechanical properties and fracture characteristics of as-cast AlMg5Si2Mn alloy was studied. The results show that adding appropriate Sc have significant deterioration effects on primary α-Al and eutectic Mg2 Si in AlMg5Si2Mn alloy. With the increase of Sc content from 0 to 0.25%, the morphology of primary α-Al changes from large block to small ball, and the morphology of eutectic Mg2 Si changes from Chinese character to fiber or dot. Sc forms the Al3 Sc phase in the alloy and as a heterogeneous nucleated substrate to promote primary α-Al nucleation and grain refinement. Sc atoms enrich in the growth front of eutectic Mg2 Si phase form subcooling components, and the growth of Mg2 Si phase is inhibited. The addition of Sc can improve the tensile strength and elongation of the alloy. Compared with the undeteriorated alloy, the alloy with 0.15%~0.25%Sc has the best tensile properties, and the tensile strength and elongation are improved by 20.9% and 60.4%, respectively.