Abstract: Objective The aim is to realize high-quality welding of die-cast magnesium alloy, it is the key to expand the application of magnesium alloy in the industrial field. Methods In order to improve the quality of conventional AC tungsten inert gas welding (TIG) joint of die-cast magnesium alloy, ultrasonic electric signal is used to excite arc and generates ultrasonic acting on arc and molten pool. The influence of ultrasonic arc on the forming, metallographic structure and mechanical properties of AZ31B die-cast magnesium alloy flat butt welded joint are compared studied. Results The results show that ultrasonic arc can improve the weld forming, refine the weld microstructure and increase the strength and plasticity of the welded joints. Compared with conventional TIG, applying ultrasonic excitation can extend the middle width and lower width of weld, making the joint surface and boundary melt evenly, the weld grain is refined. At the same time, the aggregation and growth of the second phase are inhibited, and the uniform distribution of the second phase in the weld is promoted. The average tensile strength of the joint is 212 MPa, reaching 80.9% of the base material, and the elongation after fracture increases from 5.1% to 8.5%. The joint tensile specimen without ultrasonic arc fracture in the weld. The joint tensile specimen with ultrasonic arc fracture in the heat affected zone, whose fracture morphologies are characterized by quasi-cleavage fracture. Conclusion Ultrasonic arc can help improve the welding quality of die-cast magnesium alloys and significantly enhance the mechanical properties of welded joints.