The diffusion welding between aluminum alloy and steel was achieved in this study. The influence of welding temperature on the interfacial microstructure and tensile shear strength of the joint were investigated. The joint with 101.3 MPa was obtained under the condition of the welding temperature of 525 °C. The results reveal that the reaction layer thickness increases with the increasing of welding temperatures and that the reaction layer consists of Al5Fe2 and Al13Fe4 formed in the interface. The strength of the joint is related to the reaction layer thickness and when the reaction layer thickness was approximately 0.8 μm, the value of the joint strength reached maximum.