Abstract: In order to analyze the evolution of microstructure and mechanical properties of magnesium alloy welded joints under the combined effect of magnetic field and activated flux, longitudinal AC magnetic field was used during A-TIG welding of AZ91 magnesium alloy. The formability, microstructure, phase composition and mechanical properties of welded joints with different coating amounts of activated flux were analyzed, the arc shape and crystallization nucleation characteristics under the combined action of magnetic field and activated flux were discussed. The experimental results show that the introduction of magnetic field has a negative effect on increasing penetration and improving welding efficiency, but it has a very obvious effect on improving the mechanical properties of welded joints. When the coating amount of activated flux is 3 mg/cm² under the selected magnetic field parameters, the forming state and microstructure performance of welded joints reach the best match. In this condition, the forming coefficient is 2.38, and the tensile strength and elongation of welded joints are 338 MPa and 13.3%, respectively. Under the combined action of magnetic field and activted flux, the arc moves downward spirally, and drives the molten pool to change the crystal crystallization conditions, promotes the formation of small equiaxed crystals and the appearance of twins, so that the mechanical properties of welded joints are improved. At the same time, the introduction of magnetic field can change the growth mode of the crystal, and the preferential growth phenomenon appears along the (0001) crystal plane.