The steel and aluminum alloy plates were joined by refilling friction stir spot welding. The interface morphologies of steel/aluminum spot-welded joints were observed using equipment such as optical microscope, scanning electron microscope, and X-Ray electron probe microanalyzer, and the correlation mechanism between the mechanical properties and the interface morphologies of the steel/aluminum spot-welded joints were analyzed. The results show that the fracture type of the steel/aluminum spot-welded joint is button fracture, and the mechanical properties are good. The steel/aluminum spot-welded joints are well bonded, and no obvious welding defect is found. Hook and vortex-like structures can be observed at the interface of steel/aluminum spot-welded joints, which provide mechanical interlocking effects and are conducive to the bonding of the base material. A intermetallic compound layer with the main elements of Al, Fe, and Si forms at the interface of steel/aluminum spot-welded joints. Effective metallurgical and mechanical bonding are the main reasons for the good performance of steel/aluminum spot-welded joints.