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