The invention relates to an automobile sill beam aluminum alloy and a preparation method of the automobile sill beam aluminum alloy. The method comprises the following steps of (1) in the process of smelting molten aluminum, adding an aluminum-copper alloy, an aluminum-silicon alloy, magnesium ingot and a manganese agent, wherein the mass percentage of added elements in a final aluminum rod is asfollows: 0.90-1.10wt% of magnesium, 0.90-1.10wt% of silicon, 0.50-0.80wt% of manganese, and 0.20-0.40wt% of copper; (2) filtering an aluminum liquid; (3) performing continuous casting through a casting system to obtain an aluminum rod; (4) transferring the aluminum rod into a homogenizing furnace for homogenization under the temperature of 550-570 DEG C, performing heat preservation for 8-12 h, and carrying out air mist cooling; and (5) sawing the aluminum rod to obtain short rods for extrusion. By adjusting the magnesium-silicon ratio, the content of Cu is increased, the product performance is further improved, and meanwhile, it is ensured that the decay rate can be low during later-period baking, and the performance of the prepared aluminum alloy in different states can meet requirements.