In order to improve the performance of aluminum/steel connection, an unequal thickness aluminum/steel pair welding head whose tensile strength is 40% of that of the aluminum alloy base metal was obtained using the laser technology by adjusting the process parameters. The joint weld structure, interface compounds, and mechanical properties were analyzed. The results show that the welding speed is 1.8 m/min, the laser is biased to the steel side by 0.3 mm, the defocusing amount is 0 mm, the laser power is 3.0 kW, and the tensile strength of the joint reaches 38 MPa, respectively. With the other welding parameters unchanged, when the defocus amount is -2 mm, the tensile strength of the joint is further increased to 57.7 MPa, the shape of the weld section changes from wine cup shape to corset waist, the fusion line is more neat, and the cracks and porosity defects near it are significantly reduced. Phase diffraction and energy dispersive spectrometer in the interface area of the weld showed that the compounds grown vertically along the fusion line were FeAl, FeAl3, Fe2Al5 and Fe2CrAl and Fe3Al with different degrees of brittleness. The overall tensile fracture mode of the joint is brittle fracture, the connection strength at some positions on the fracture surface is good, showing a concave shape, and Fe3Al is found through inspection Fe3Al. The results of this study are improving the performance of aluminum/steel joints in the bodywork, weight reduction and energy saving and emission reduction are of great practical significance.