A three-dimensional micromechanical model for pseudoelasticity is implemented into ABAQUS to study the mechanical behavior of a polycrystalline NiTi shape memory alloy under biaxial loading. The model is firstly validated by numerical method and then used to simulate a thin-wall tube under non-proportional extension-torsion loading. When the tensile strain remains constant, the tensile stress decreases with increasing of the shear strain. While unloading the shear strain, the tensile stress increases. This is consistent with experimental results. The model can be used to get an idea of the pseudoelastic behavior of NiTi alloys under complex stress states.