Abstract: In order to verify the feasibility of friction stir spot welding of Q235 carbon structural steel and 6061 aluminum alloy,the Q235 steel sheet with thickness of 2 mm as upper sheet and the 6061 aluminum alloy plate with thickness of 5 mm and 10 mm as lower plate were welded respectively under the test conditions of spindle speed 1200 r/min,axial load of 12 k N and different extrusion time.The results show that for the 5 mm thick aluminum alloy plate,the temperature of the steel/Al interface rises rapidly to the liquid-phase line temperature （652℃） of the 6061 aluminum alloy at 8 s,and it reaches 740℃ at 15 s,and the failure load of the weld seam is 10-15 k N after welding;for the 10mm-thick aluminum alloy plate,the interface temperature rises to 652℃ at about 20 s,and to 740℃ at 45 s,the failure load of the weld seam reaches 18-22 k N.The central area of aluminum alloy plate in the interface of Q235 steel/6061 aluminum alloy melts to form a cladding metal layer containing iron and aluminum,which is Fe₂Al₅ and Fe₄Al₁₃determined by XRD.It is found that the thickness of cladding metal layer of the 5 mm-thick aluminum alloy plate at the joint surface is thicker than that of the aluminum alloy plate with 10 mm thickness analyzed by using SEM,and the thicker cladding metal layer is easier to form crack,thus weakens the failure load.