The influences of external longitudinal magnetic field on weld joint forming characteristics, microstructure and microhardness distribution are investigated in SUS316L austenitic stainless steels by laser-metal inert gas hybrid welding. The experimental results show that, under the action of external longitudinal magnetic field, the weld joint reinforcement decreases, the weld width increases, the forming coefficient increases, and the cross section appears wide and deep. The external magnetic field alters the thermal cycle of the weld joint, which makes the striped δ-ferrite precipitation occur in the heat affected zone and the grain growth is inhibited. The external magnetic field causes the rotation of the molten pool and the weld joint grains are refined. The uniformity of crystallization is improved and the microhardness distribution becomes stable. This effect enhances with the increase of the magnetic induction intensity but weakens with the increase of the weld joint depth.