The paper aims to explore effects of ultrasonic vibration on friction stir welding of aluminum alloy. Microstructure, mechanical property and fracture surface of 7075-T6 aluminum alloy were analyzed through ordinary friction stir welding and ultrasonic-assisted friction stir welding respectively. A tunnel-type defect was generated in the ordinary friction stir welding. After the ultrasonic vibration was applied, the defect disappeared and a good joint without defect was formed. Compared with the ordinary friction stir welding, the degree of growth of heat affected zone grains welded by ultrasonic-assisted friction stir welding was small, the grain size of the weld nugget was refined, the joint strength was obviously improved, and it reached 71.5% of the strength of the aluminum alloy base metal. The joint fracture mode was a mixed fracture mode of dimple and quasi-cleavage. Ultrasonic vibration promotes the flow of plastic metal, which can effectively inhibit the formation of holes, tunnel-type defects, etc. At the same time, ultrasonic vibration can increase the plasticity of the metal and reduce the heat input of the weld.