Abstract: The laser lap welding experiment of steel on aluminum alloys was carried out by using the 8-shaped oscillation mode. Metallographic microscope, scanning electron microscope and electronic universal testing machine were used to analyze the influence of oscillation frequency on the macroscopic morphology, microstructure, fracture characteristics and mechanical properties of the joint. The mechanism of changes in weld formability, morphology and stability of molten pool, and mechanical properties of joint under different oscillation frequencies were discussed by using high-speed cameras and simulation calculations. The results show that the formability of the weld surface and the stability of the molten pool flow significantly decrease with the increase of oscillation frequency, and there is a direct correspondence between the morphology characteristics and energy density distribution of the molten pool. During low-frequency oscillation, the thickness of the IMC layer at the interface is small and the uniform distribution of the Al element in the molten pool has a solid solution strengthening effect. During high-frequency oscillation, the strong vortex effect causes the Al element to aggregate at the interface, leading to an increase in the IMC layer thickness at the interface, and the increase of the layered phase thickness is the main reason for the decline of the shear tensile performance of the joint. Compared with the non-oscillation mode, the maximum shear tensile force and elongation of the joint are increased by 31.5% and 60%, respectively. The 8-shaped oscillation can significantly improve the strength and toughness of the joint by regulating the thickness of the interface IMC layer.