The invention discloses magnesium aluminum alloy and a preparation method thereof. The magnesium aluminum cast alloy comprises, by weight, 7-12% of aluminum, 0.1-0.8% of manganese, 0.1-1.0% of cerium,3-6% of chromium, 0.2-0.4% of zinc, 0.5-1.2% of strontium, 0.7-1.0% of titanium and the balance magnesium and inevitable impurities. The impurities comprises, by mass, 1-3% of cobalt, less than or equal to 5% of tungsten, less than or equal to 2% of niobium, less than or equal to 2% of vanadium. The content of magnesium is no less than 65%. According to the magnesium aluminum alloy preparation method, through melting, refining and liquid surface scum removing, a mixed melt is fed into an intermediate frequency furnace and deposited on a substrate after atomizing through high-pressure argon, alloy ingots are formed, then the alloy ingots are cooled to the room temperature, and a finished magnesium aluminum alloy product is obtained. According to the technical scheme, the magnesium aluminumalloy has good mechanical properties and structure uniformity, the corrosion resistance is improved, the material utilization rate is high, the yield of products is increased, and the production costis reduced.