2024-T4 aluminum alloy plate was jointed by friction stir welding (FSW) under air and cooling water circulation separately, and the effect of the cooling water on the microstructure and mechanical properties of the FSW joint was investigated. The results show that the cooling water circulation medium has a significant instantaneous cooling effect. Cooling water causes remarkable grain refinement with an average grain size of 700 nm in the nugget zone. The growth of precipitates is restrained by cooling water, and the size of precipitates is 30~200 nm in the nugget zone. Cooling water decreases the thermal softening effect on the FSW joint, thus improving the microstructure and properties of the joint. HV microhardness and tensile strength of SFSW joint are increased by 234 MPa and 52.2 MPa, respectively, but the elongation is decreased a little