Abstract: In orde to study the effects of welding parameters on mechanical properties and the microstructure of refill friction stir spot welds between alclad 2A12 Al alloy and 2219 Al alloy, the mathematical model between tool plunge depth, rotational speed, plunge speed and the shear strength of the joint was established based on response surface method. The results show that the plunge depth is the most significant parameter that affects the weld strength. In the weld microstructue, the alclad layer is mainly gathered at residual binding line of the joint, while part of the alclad layer is distributed along the sleeve path in vertical direction. The welding parameters affect morphologies of residual binding line, and thereafter the strength and fracture mode of welds. Interfacial fracture mode occurs in most of welds due to the low strength of pure aluminum gathered at the residual binding line. While the weld produced at the plunge depth of 1 mm, rotational speed of 2250 r/min, plunge speed 0.5 mm/s shows pull-out fracture mode, because the strength of the retract line region is weaker than that of the interface, but it does not have the maximum failure load.