The 7475 aluminum alloy backfill friction stir spot welding is studied in this paper. Firstly, the 2mm 7475 aluminum alloy is welded by different process parameters, then the surface and cross section of the joint are observed, and the hardness and tensile shear test of the sample are carried out. The results show that the joint can be divided into four parts according to the weld microstructure, and the grain size of different parts is different. Hook, holes and unjointed microstructures due to poor fluidity of the material are found in the microstructure. At the same time, the hardness of the base metal zone and stirring zone is significantly higher than that of the heat affected zone and the thermo-mechanically affected zone; in addition, the highest hardness and tensile shear strength of the joint vary with welding tool rotation speed land the depth of the sleeve. However, the effect of the stirring sleeve movement rate on the mechanical properties of the solder joint is not obvious.