The microstructure and mechanical properties of Al-10.5Si-5Zn-xMg alloys with different Mg contents were studied by optical microscopy (OM), scanning electron microscopy (SEM), differential scanning calorimeter (DSC) and tensile test. The results show that when Mg content increased from 0 to 0.8%, the area fraction of α-Al dendrite in the alloy increased slightly, which promoted the precipitation of a small amount of massive silicon phase, and the number and size of the dendrite gradually increased. The iron-rich phase changes from tiny, compact, granular α-Fe and acicular β-Al5 FeMnSi to a thick, skeletal Al15 (Fe,Mn)3 Si2 phase. Moreover, π-Al8 Mg3 (Fe,Mn)Si6 iron-rich phase similar to the color of eutectic silicon appeared after adding more than 0.35% Mg. However, the area fraction of iron-rich phase did not change significantly with the increase of Mg content. In addition, the increase of Mg content also promotes the precipitation of primary Mg2 Si phase, which change from the shape of short stick to the shape of thick Chinese characters. The alloy has the best comprehensive mechanical properties when Mg content is 0.5%, and the tensile strength, yield strength and elongation are 265 MPa, 204 MPa and 4.8%, respectively.