Abstract: Objective The connection of aluminum alloy and steel is widely used in the fields of transportation, construction and electronic appliances, and welded joints are prone to porosity, cracks and other defects during welding, which leads to deterioration of performance of aluminum/steel composite structure. In this paper, friction stir welding is carried out on 6061 aluminum alloy/304 stainless steel with copper foil to study microstructure and mechanical properties of welded joints. Methods By theoretical analysis, the addition material is selected to test and to prepare welding samples. The interface structure and mechanical properties of welded joints at different welding speeds are studied by means of metallographic observation and tensile test. Results The results show that there are differences in metallurgical reaction, dispersion strengthening of copper particles and Hook size at the interface of welded joints at different welding speeds. Under appropriate welding parameters, the main factor affecting bonding strength of friction stir lap joints of dissimilar metals is metallurgical reaction at the interface, followed by Hook size and particle dispersion strengthening and other mechanical mixing. When welding speed is 10 mm/min, samples break at the interface. When welding speed is 20～40 mm/min, samples fail in aluminum alloy on the retreating side. The maximum tensile load of 2.16 kN is obtained at the welding speed of 20 mm/min. When no copper foil is added, samples fail at the interface, and tensile load is only 0.56 kN. Conclusion Friction stir welding process could be used to connect 6061 aluminum alloy/304 stainless steel with copper foil. When rotation speed is 1 200 r/min, welding speed is 20 mm/min, and down pressure is 0.1 mm, high-quality welded joints can be obtained.