The invention provides a local vacuum casting method for aluminum alloy. A raw material is sequentially added step by step, and ultrasonic vibration treatment and mechanical stirring treatment are combined, so refinement of a melt structure can be remarkably promoted, the dendritic crystal spacing is reduced, and the dendritic crystal spacing is stable; an alloy casting is formed under the actionof vacuum and pressure formed at the upper part of a crystallizer through the local vacuumizing casting method, the grain diameter is small, the structure is compact, and the hole defects are few; theconnection among a filter chamber box, a molten aluminum inlet and a crystallizer is always kept in a vacuum state, so the situations of segregation, turbulence, easy gas entrapment and oxidation ofthe aluminum alloy in the production process are reduced, the shifting amplitude of the liquid level of the aluminum alloy is stable, and the slag entrapment phenomenon can be obviously improved; andthe local vacuumizing casting method for aluminum alloy has the advantages of simple device, and easiness in operation, and the prepared aluminum alloy has no obvious defects such as cracks, pits andslag inclusion on the surface, and has the advantages of large tensile strength, good yield strength and high extrudability.