Abstract: To improve the mechanical properties of aluminum alloy and steel joints and suppress the excessive formation of brittle intermetallic compounds (IMCs) at the interface during laser welding, AlSiCu alloy powder and composite powder prepared by AlSiCu + Mo powder were employed as an interlayer for deep penetration welding with laser lap of 6016 aluminum alloy and DC06 low-carbon steel, respectively. The results indicate that with the process parameter conditions of laser power of 900 W, welding speed of 0.07 m/s, and defocus distance of +2 mm, after adding either AlSiCu powder or AlSiCu + Mo composite powder to the interlayer, the welding process is stable, and the weld formation is sound. Both the penetration depth and weld width on the aluminum side increase compared with those obtained without powder. Moreover, with the addition of AlSiCu + 3% Mo composite powder, the grain structure is refined; brittle Fe-Al IMCs are reduced, and the sizes of layered and acicular phases at the interface are decreased. The tensile-shear force of the aluminum/steel welded joint reaches 82.3 N/mm, which is 25.3% higher than that of the joint without powder addition, and the fracture occurs in the heat-affected zone of the aluminum alloy and part of the aluminum base metal.