For overcoming the shortcomings of traditional welding methods, laser-induction hybrid welding (LIHW) was investigated. In this paper, high strength low alloy steel S690QL was selected to explore the improvement mechanism of LIHW on weld formation, mainly through high-speed photography, optical microscope and electron back-scattered diffraction (EBSD) techniques to compare the vapor/plasma plume and keyhole dynamic behaviors of laser beam welding (LBW) and LIHW. Both the characteristics of weld joint geometry and microstructure processed by LIHW technology were better than LBW. With increasing inductor output power, the changing period of vapor/plasma plume was accelerated, the spatter defects were inhibited, the molten pool length and width increased. At the same time, due to the better welding heat input absorption and utilization, most the LIHW weld joints contain more ferrite, lower bainite and martensite than LBW weld joint. Furthermore, the alternating magnetic field and the two eruption paths of upper keyhole and lower penetrated hole in LIHW processing could lead to the penetration of LIHW joints, while the LBW does not. Thus, the weld formation mechanism was revealed and compared with LBW, LIHW have better molten pool and vapor/plasma plume dynamic behaviors, which result in the improvement of weld formation.