Abstract: Extruded AZ31 magnesium alloy bars are widely used as lightweight shaft and torque-bearing components in automotive, railway and aerospace structures, where they are frequently subjected to combined tension-torsion loading. However, the influence of such loading paths on the stress–strain response, deformation mechanisms and fracture modes remains insufficiently understood. In this study, extruded AZ31 magnesium alloy bars are investigated under a series of loading paths with stress path angles (α) of 0° (uniaxial tension), 15°, 30°, 45°, 60°, 75° (tension-torsion loading) and 90° (free-end torsion). Macroscopic mechanical tests, microstructural characterization and fracture surface observations are conducted, and a torsion-specific finite element (TFE) method based on the EVPSC-TDT (elastic-viscoplastic self-consistent coupled with twinning-detwinning scheme) model is applied to describe the multi-axial response under combined tension-torsion, and systematically reveal the effects of complex stress states on the alloy’s mechanical behaviors and fracture mechanisms. The numerical results capture well the equivalent stress-strain curves and texture evolution along different tension-torsion paths, demonstrating that the proposed model can effectively represent the coupled tension-torsion deformation behavior of extruded AZ31 bars. By further combining global Schmid factor analysis, it is revealed that with increasing stress-path angle and the corresponding Lode stress parameter, the dominant plastic deformation mechanism gradually shifts from prismatic slip to basal slip; under loading paths with a large shear component, extension twinning is markedly activated, introducing new grain orientations and weakening the initial extrusion texture. Fractographic analysis shows that as the tension-torsion ratio evolves from tension-dominated to torsion-dominated, the fracture mode changes from a microvoid-coalescence ductile fracture with large and deep dimples to a mixed ductile–brittle fracture with fine dimples and quasi-cleavage facets, and finally to a fracture morphology dominated by cleavage-like features. These results provide experimental evidence and theoretical support for multi-axial constitutive modeling and fracture-safety design of extruded AZ31 magnesium alloy bars under complex tension-torsion service conditions.