Abstract: Laser lap deep penetration welding was conducted on Al/steel dissimilar metals with longitudinal alternating magnetic field. Effects of alternating magnetic field on the macro-morphology, microstructure, element distribution, mechanical properties and fracture mode of the joints were carefully investigated using metallographic microscope, scanning electron microscope and other methodologies. The results showed that the weld width decreased and the weld depth increased under applied alternating magnetic field. The electromagnetic force induced by an alternating magnetic field promoted the transformation of resultant IMCs at interface from continuous distribution to discrete distribution. During this process, composition segregation and crack defects were also successfully suppressed. IMCs in interfacial layer may be composed of ζ-FeAl₂, θ-FeAl₃ and β₂-FeAl. Laser welding with applied external magnetic field can improve the loading capacity of the joint. It can attribute to the increase of combined areas and refinement of IMCs structure.