Abstract: FSSW without keyhole of Al6061 and DP600 zinc-coated steel was carried out by using the equipment designed by our team (top steel sheet-bottom Al sheet). Using scanning electron microscope, energy spectrometer and tensile test on the microstructure and mechanical properties of joint were studied. The results showed that the mixing head speed was 1200 r/min, the preheating time was 6 s and stirring needle length was 3.2 mm and joint shear resistance reached 11.2 kN. The surface of joint was smoothly without keyhole. Welding joint was composed of the stir zone and the diffusion zone, stir zone to hook shape the distribution of embedded steel in the aluminum matrix and finally form strong mechanical connection. The diffusion zone of aluminum and steel's connection depended on the interfacial intermetallic compounds. Connection mode was metallurgical combination. The fracture of welding joint's position was in between of the interface layer and the aluminum substrate. The distribution of river-shaped fracture was cleavage steps, so the mode of fracture belonged to brittle fracture.