Lightweight is the general trend of the development of the automobile industry. As a non heat treated aluminum alloy, the Al-Mg-Si aluminum alloy has great application potential in lightweight integrated forming process. Although the casting performance is lower than that of Al-Si alloy, the Al-Mg-Si alloy has higher strength in the as cast condition. Due to the high solid solubility of Mg in Al, the Mg content is crucial to the strength of the Al-Mg-Si alloy. The influence of Mg addition on the microstructure and mechanical property of the Al-Mg-Si alloy is studied in gravity die casting. The introduction of Mg element leads to the increasing volume fraction of Al-Mg2Si eutectics and β-AlMg phases in microstructure, which greatly increases the strength of Al-Mg-Si alloys. The yield strength and ultimate tensile strength of alloy are enhanced by 62% and 11% when the Mg content increases from 7.6 wt pct to 11.3 wt pct in Al-xMg-2.6Si alloys. When the Mg addition is 11.3 wt pct, the YS and UTS of Al-11.3Mg-2.6Si alloy are 146 MPa and 228 MPa, respectively. While the increasing volume fraction of Al-Mg2Si eutectics and β-AlMg phases precipitated in grain boundary results to the decreasing ductility of alloy, the elongation (EL) is decreased by 41% with increasing Mg content from 7.6 wt pct to 11.3 wt pct. The EL of Al-11.3Mg-2.6Si alloy is 2.0% in the as cast condition.