The microstructure and mechanical properties of 0.6 mm thick LY12 aluminum alloy were studied by using the refill friction stir spot welding. According to the characteristics of welded joints,it is divided into the stir zone,thermo mechanically affected zone,heat affected zone and base metal. The results showed that there was no obvious deformation of the specimen after welding,the stir zone formed fine equiaxed grain,TMAZ formation of fibrous tissue,some grains during recovery and recrystallization,the stir zone of maximum hardness,the minimum value of the thermo mechanically affected zone,the rotational speed and welding time increased respectively,and the shear force increased first and then decreased,the shear fracture mode was brittle fracture,the crack started at the boundary and expanded along the thickness of the plate,the distribution of dimple tissue was uniform at the center of fracture,the optimum welding parameters for rotating speed 2 000 r/min,press amount 0.2 mm,the welding time of 3.5 s with the depth of 0.8 mm.