Morphology, distribution, composition, forming ability, structural stability and intrinsic mechanical properties of the intermetallic compounds (IMCs) formed in steel/aluminum laser welding were determined through scanning electron microscope, energy dispersive spectrometer, X-ray diffractometer and first-principles calculation. It was found that the mechanical properties of the joint are limited by the Fe-Al IMCs, whose brittleness is attributed to the orbital hybridization between Al(s), Al(p) and Fe(d). However, the joint properties are improved by adding interlayer, which is ascribed to some changes of electronic structure of the generated IMCs. The transition mechanism of IMCs changing from brittle to ductile is mainly due to the weak ability of interlayer elements to attract electrons. The mechanical properties of the joint are closely related to the ductility or brittleness of the IMCs. Moreover, the addition of Ti foil interlayer effectively improves the mechanical properties of the joints, which means that the experimental verification is in good agreement with the theoretical calculation predictions.