The invention discloses a magnesium-containing Al-30Si aluminum alloy and a preparation technique thereof. The aluminum alloy is prepared by pouring an aluminum alloy molten mass with 750 DEG C into a metal mold with 200 DEG C in an atmospheric environment. The Al-30Si mother alloy is prepared by the following steps: smelting industrial pure aluminum with the purity of higher than 99.8 Wt% and silicon metal with the purity of higher than 99.5 Wt% in a graphite crucible by using a well-type resistance furnace, keeping the temperature at 900 DEG C for 4 hours, carrying out refining and degassing on the alloy liquid with hexachloroethane at 750 DEG C, and keeping the temperature for 30 minutes. The element magnesium is alloyed by adding magnesium ingots with the purity of higher than 99.9 Wt% at 720 DEG C and keeping the temperature for 30 minutes. The aluminum-silicon alloy is composed of primary silicon, eutectic silicon, alpha-Al, Mg2Si and other phases, and comprises the following chemical components: 30.0 Wt% of Si, 0.4-1.6 Wt% of Mg and the balance of Al. The tensile strength is 124.1-218.4 MPa, the hardness is 58.9-108.8 HBW, and the resistivity is 0.35-0.37 ohm m.