Abstract: Objective To address the common issues of coarse weld grains and the difficulty in balancing strength and ductility in TIG welded joints of AZ80 magnesium alloy, this study explores an effective welding method that regulates the solidification microstructure of the weld via a hybrid energy field. Methods A self-developed ultrasonic-frequency pulse-modulated TIG welding system was used to introduce a 50 kHz ultrasonic-frequency pulsed electrical signal into the TIG arc for fusion welding of AZ80 magnesium alloy. The weld characteristics were characterized by macroscopic morphology observation, XRD, optical microscopy, SEM, and EBSD. Combined with tensile testing and fracture morphology analysis, the effects of ultrasonic-frequency pulses on the weld formation, microstructure, and mechanical properties of the welded joints were comparatively investigated. Results The results show that compared with conventional TIG welding, the weld penetration of the ultrasonic-frequency pulse-modulated TIG welded joint increases from about 6.6 mm to about 9.6 mm, with a slight increase in weld width, and the weld formation is significantly improved. The phase composition of the weld remains unchanged, consisting of α-Mg and β-Mg₁₇Al₁₂ phases, but the weld microstructure is significantly refined, with a noticeable reduction in columnar grains and an increase in the proportion of equiaxed grains. EBSD analysis reveals that the maximum grain size in the weld decreases from about 100 μm to about 60 μm, and the average grain size decreases from about 30.2 μm to about 18.8 μm, a reduction of approximately 38%. The ultrasonic action promotes dendrite fragmentation and uniform nucleation in the molten pool, while also making the Al element distribution more uniform. Mechanical property test results indicate that the tensile strength of the welded joint increases from about 204 MPa to about 248 MPa, an improvement of about 22%, with a simultaneous increase in elongation after fracture, and the fracture mode transitions from brittle fracture to ductile fracture. Conclusion The ultrasonic-frequency pulse-modulated TIG welding, through the hybrid energy field mechanism of “arc contraction-acoustic streaming/cavitation in the molten pool,” enhances welding melting efficiency and effectively refines the solidification microstructure of the weld. It achieves synergistic improvement in both strength and ductility of AZ80 magnesium alloy TIG welded joints, providing a new process approach for high-quality arc welding of magnesium alloys.