In order to obtain high quality steel/aluminum joints, the lapping experiments of DP780 duplex steel and 5083 aluminum alloy with different power were carried out by laser swing welding. The effects of different welding power on the macroscopic morphology, microstructure, and mechanical properties of steel/aluminum joints were investigated. The test results show that the plate welding can be effectively realized within the power range of 1400W~1600W. When the laser power is 1400W and 1500W, the metallographic structure of the welded joint is dominated by martensite. When the laser power is 1600W, the amount of ferrite in the joint increases, while the martensite decreases, and the metallographic structure of the welded joint is dominated by ferrite. The lowest and highest microhardness of the three joints were located in the weld zone and the heat affected zone, respectively. Under the laser power of 1500W, the mechanical properties of the welded joint are the best, and the microhardness of the steel side joint is about 1.7 times higher than that of the base metal. The maximum shear strength of the joint is 113N/mm. The research results are of great significance in the field of shipbuilding.