In the past decade, a unique composite system consisting of Nb nanowire and NiTi shape memory alloy matrix has attracted considerable attention. One of the works published in Science proposed that the NiTi-Nb composite has superior properties, including high strength (1.65 GPa), low Young's modulus (25.8 GPa), and quasi-linear superelasticity (6.4%). In particular, given the quasi-linear superelasticity of this composite, (1) continuous stress-induced martensitic transformation occurred even at the beginning of tensile loading, which indicated that the external stress required to start the transformation was reduced to almost zero; (2) the transformation (stress-strain) curve is a “hardening type” rather than a “plateau type,” with apparent Young's modulus of 25.8 GPa, and (3) the amount of quasi-linear superelasticity deformation is 6.4%, which is higher than that of conventional binary NiTi alloy. This work focused on the quasi-linear superelasticity property. Thus, an in situ NiTi-Nb composite was prepared by vacuum induction melting, hot forging, and wire drawing. Microscopic analysis showed that Nb nanowires were distributed in parallel inside the nanocrystalline NiTi matrix along the wire axial direction. Quasi-linear superelasticity was obtained after 9% pre-deformation, with a yield stress of 1.7 GPa, apparent Young's modulus of 34 GPa, and quasi-linear superelasticity deformation of ~5.5%, which is similar to the result proposed in Science. In situ synchrotron XRD measurements were conducted to analyze the effect of pre-deformation on the coupling effect between NiTi and Nb nanowire. The origin and deformation mechanism of the quasi-linear superelasticity were systematically studied.Results revealed that coupling tensile stress in NiTi, which was generated by pre-deformation, increased gradually with the increase of the pre-deformation strain, thereby providing a driving force for stress-induced martensitic transformation. The external stress required to start the transformation could be reduced to almost zero in some local areas as a result of the coupling tensile stress. The initial velocity of transformation increased with the increase of the coupling tensile stress in NiTi. Therefore, a continuous transformation with relatively high velocity was obtained even at the beginning of tensile loading after a proper pre-deformation. Furthermore, the gradient distribution of coupling tensile stress inside B2-NiTi led to the “hardening-type” transformation (stress-strain) curve.