The mechanisms responsible for deformation behavior in Nb/NiTi composite during pre-straining were investigated systematically using in-situ synchrotron X-ray diffraction, transmission electron microscopy and tensile test. It is shown that upon loading, the composite experiences elastic elongation and slight plastic deformation of B19′, B2 and β-Nb phases, together with the forward stress-induced martensitic (SIM) transformation from B2 to B19′. Upon unloading, the deformation mechanisms of the composite mainly involve elastic recovery of B19′, B2 and β-Nb phases, compression deformation of β-Nb phase and incomplete B19′→B2 reverse SIM transformation. In the tensile loading-unloading procedure, besides the inherent elastic deformation and SIM transformation, the (001) compound twins in B19′ martensite can also be conducive to the elastic deformation occurring in B19′-phase of the composite. Therefore, this composite can exhibit a large recoverable strain after unloading owing to the elastic deformation, and the partially reversible and consecutive SIM transformation together with the (001) compound twins.