Abstract: 2024-T3 high-strength aluminum alloy and Q235 steel have significant application value in lightweight structural manufacturing. However, there are significant differences in their density, melting points, and thermal expansion coefficients, presenting numerous challenges for their joining. The friction stir welding (FSW) process of 2024-T3 aluminum/Q235 steel dissimilar metals lap joints was investigated using different stirring head rotational speeds, and the microstructure and mechanical properties of the joints were analyzed. The results show that "hook-like" structures are observed in the stirring zone on both the advancing and retreating sides of the joint. The structure of the stirring zone is fine equiaxed grains, plastic deformation and local recrystallization occur in the TMAZ metal, and the grains in the HAZ grow significantly. A hard and brittle intermetallic compound (IMC) layer is formed at the aluminum/steel interface, and the thickness of the IMC layer increases with the increase of rotational speed. When the rotational speed is 1000 r/min, the effective sheet thickness of the joint is minimum. The tensile-shear strength of the joint is optimal, reaching 37.6 MPa. There is aluminum-steel intermetallic compound on the tensile shear fracture surface, which belongs to brittle fracture.