The　welding　characteristics　of　5052　aluminum　alloy　and　Q235　low-carbon　steel　sheet　were　systematically　studied　by　the　refilled　friction　stir　spot　welding.　The　effects　of　rotation　speed　and　pressure　speed　on　weld　forming,　tensile　strength,　and　welded　joint　structure　were　analyzed　in　different　welding　modes.　The　results　indicated　two　different　connection　modes:　the　chimeric　mode　and　the　non-chimeric　mode.　The　corresponding　depression　depth　are　2　and　2.4　mm,　respectively.　In　the　non-chimeric　connection　mode,　the　steel/aluminum　metallurgical　interface　is　a　smooth　transition,　the　hook　structure　is　obvious,　and　the　welding　mechanism　mainly　depends　on　the　mutual　diffusion　between　atoms.　However,　in　the　chimeric　mode,　a　hook　structure　will　be　formed　at　the　metallurgical　interface　of　steel　and　aluminum.　The　connection　mechanism　is　determined　by　mechanical　interlocking　and　mutual　diffusion.　The　maximum　strength　value　is　7.48　kN　in　non-chimeric　mode.　At　this　time,　the　spindle　speed　is　1300　r/min　and　the　pressure　speed　is　1　mm/s.　There　were　two　types　of　fractures:　the　button　fracture　mode　and　the　peel　fracture　mode.