Large plastic deformation commonly occurs during the practical forming process in industries. Compared with uniaxial tension/compression, torsion is a more effective approach to investigate mechanical behavior under large deformation. The response of the large strain torsion of magnesium alloy is sensitive to the initial texture and twinning. Therefore, an extruded AZ31 alloy was pre-stressed in tension and compression to introduce dislocations and twins in the current work. Subsequently, torsion tests were conducted to clarify the effects of twinning and dislocation on subsequent deformation responses. The corresponding microstructure and deformation mechanisms were explored on the basis of viscoplastic self-consistent (VPSC) modeling. The experimental observations on stress–strain responses and pole figures were captured by simulation work. It was found that twins make less contribution to plastic deformation, which results in small change in texture direction under pure torsion and torsion after pretension. The activity of the slip/twin system and the mechanical properties are affected by different initial textures and active conditions of the system. Moreover, the stress state during combined tension–torsion loading benefits the reduction of texture intensity.