Abstract: Aluminum alloy 2219-T351plates were welded by friction stir welding and ultrasonic assisted friction stir welding with different power. The welding temperature and welding pressure were investigated. Microstructure, microhardness and mechanical properties of welds under different ultrasonic powers were analyzed. The microstructure and material fluidity of the weld with different ultrasonic power were researched. Experimental results showed that Ultrasonic energy can reduce the welding temperature and welding force. With the action of ultrasound, the microstructure of the weld is more uniform, and the fluidity of the bottom material is improved. There is more residual reinforcement phase in the weld zone. The microhardness, tensile strength and elongation of the welded joints are all improved under ultrasonic. All the magnitudes reached the highest when adding 2.25 kW ultrasonic power. The highest tensile trength is 331 MPa, which can reach about 80% of the base metal.