The 20 mm thick 2219 C10S aluminum alloy sheets were subjected to friction stir welding. The internal defects， macroscopic morphology， microstructure， micro-hardness and tensile mechanical properties of the welding joints at the speed of 400~550 r/min were compared under the condition of a travelling speed of 100 mm/min. The results show that when the rotation speed is 400~500 r/min， the welded joints with no defects and higher internal quality can be obtained. As the speed increases， the distance between the "onion rings" in the weld nugget zone and the range of the shoulder affect zone increase， and the micro-hardness of the weld nugget zone and the thermo-mechanical affect zone decreases more obviously. The tensile mechanical property of the joints decreases with the increase of the rotation speed. The maximum tensile strength can reach 334 MPa. The fracture mode of the joint is mainly ductile fracture occurring at the junction of the thermos-mechanical affect zone and the heat affect zone on the retreating side. It coincides with the position of the minimum micro-hardness. Point defects were observed on the surface of the weld joint when the rotation speed increased to 525 r/min and continuous furrow defects were observed inside the weld joint when the rotation speed increased to 550 r/min.