Abstract: This study investigated the effects of different tool offset on material flow behavior and joint performance in friction stir welding of Al-Mg dissimilar metals. The microstructures and mechanical properties of the post-weld joints were examined and tested using optical microscopy, scanning electron microscopy, micro-hardness testing, and tensile testing. The experimental results indicate that the tool pin offset significantly influences the plastic flow and material interaction mechanism in Al/Mg dissimilar FSW joints. Specifically, as the offset increases, the total length of the Al/Mg interface decreases, and its morphology transitions from planar to a vertical/serrated shape. The grains in the nugget zone of the joint are significantly refined, with the smallest average grain size (approximately 3.04 μm) achieved at an offset of 0.8 mm. The tensile strength of the joint is minimally affected by the offset, with values of 135.7 MPa, 139.6 MPa, and 137.6 MPa corresponding to offsets of 0 mm, 0.8 mm, and 1.6 mm, respectively. However, the uniform elongation exhibits a decreasing trend with increasing offset, measured at 10.5%, 8.5%, and 8.1%, respectively. All joints fractured in the heat-affected zone on the aluminum side, with fracture surfaces displaying typical dimple morphology, characteristic of ductile fracture behavior.