The invention discloses a high-strength cast aluminum alloy which comprises the following components in percentage by weight: 6.0-8.0% of Si, 0.5-2.0% of Cu, 0.2-0.6% of Mg, 0.05-0.30% of Ti, 0.05-0.5% of Gd, 0.01-0.05% of Sr, at most 0.20% of Fe, at most 0.50% of impurity elements and the balance of aluminum. The invention also discloses a preparation method of the high-strength cast aluminum alloy. The method comprises the following steps: smelting an alloy to obtain an aluminum alloy melt, carrying out gravity casting to obtain an aluminum alloy casting, and finally, sequentially carrying out solution treatment, cooling treatment and aging treatment to obtain the high-strength cast aluminum alloy. A trace amount of rare-earth Gd element is added into the aluminum alloy components, thereby refining the crystal grains, and reducing the secondary dendrite arm spacing. The tensile strength and elongation percentage of the high-strength cast aluminum alloy obtained by preparation method are obviously enhanced.