The joining of Al/steel dissimilar metals has emerged as a critical challenge in automotive lightweight design. The sound Al/steel lap configuration joints were achieved via the new welding technology, i.e., the rotating laser welding method, with the inserted Cu foil interlayer. The interfacial temperature fields of the obtained joints were accurately predicted by finite element simulation. The results indicate that the heat concentration effect at the interface was generated via the rotating laser, and the temperature difference of the laser direct irradiation area, the middle area, and the welding toe region was controlled to 100 °C. Besides, the inserted Cu interlayer significantly optimized interfacial metallurgical reactions, resulting in the formation of a uniform composite interface layer consisting of θ-(Fe, Cu)₄Al₁₃, α-Al, and Al₂Cu. The thermodynamic calculations elucidated the sequential phase evolution: the Al-Fe binary system was generated first, and then followed by the Al-Cu binary system in the welding process. The final joint line load was enhanced with the inserted rotating laser. When the rotating diameter was 1 mm and the thickness of Cu interlayer was 10 μm, the joint line load reached the maximum value of 297 N/mm.