Abstract: To investigate the influence of Mg content on the microstructure and properties of tungsten inert gas arc welding (TIG) joints in Al-Mg-Zn alloys, three alloys, namely Al-7Mg-3Zn, Al-8.5Mg-3Zn, and Al-10Mg-3Zn, were selected for TIG welding, and the weld microstructure, crystallographic features, and mechanical properties of the joints were comparatively analyzed by combining characterization methods such as SEM, EDS, and EBSD. Results indicate that with the increase of Mg content, the second phase in the weld undergoes a significant evolution: Al₆Mn gradually refines from coarse blocks and disperses, while the Al-Mg-Zn eutectic phase gradually coarsens and is distributed continuously along the grain boundaries. EBSD characterization results show that all three welds are composed of equiaxed grains with weak textures. The 8.5Mg weld has the finest grain size (approximately 13.8 μm) and a more uniform strain distribution; the 10Mg weld exhibits significantly coarsened grains (approximately 25.6 μm), an increased proportion of low-angle grain boundaries, and localized strain concentration along the continuous second phase and grain boundaries. In terms of mechanical properties, the mechanical properties of the joints first increase and then decrease with the increase of Mg content. The 8.5Mg joint has the highest tensile strength (304.16 MPa) and the best match of strength and ductility; the strength of the 10Mg joint decreases to 242.74 MPa, and it shows a clear embrittlement tendency, which indicates that an excessively high Mg content weakens the comprehensive properties of the welded joint.