Abstract: NiCl₂ was selected as activating flux, and the longitudinal magnetic field was used in A-TIG welding process of AZ91 magnesium alloy plate. By adjusting the magnetic field current and the coating amount of activating flux their effects on the weld penetration depth, width, hardness and microstructure were studied to determine the best matching parameters. The results show that, in experiments, at the magnetic field current of I_c=1.5 A and the activating flux coating amount of 5 mg/cm², the deepest weld penetration and the highest microhardness are obtained, which can reach up to 5.00 mm and 73.8 HV, respectively. The combined action of the magnetic field and the activating flux not only increases the weld penetration depth, but also improves the mechanical properties.