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.