In the present research, to further reduce the fabrication and forming cost of NiTi alloy, an innovative wire-arc additive manufacturing process (WAAM), which is simultaneously an in-situ alloying process, has been applied to fabricate polycrystalline Ni53Ti47. The as-fabricated alloy is subsequently subjected to a necessary post-production annealing for additively manufactured metals. During the thermal cycle, to characterize the obtained phases in the Ni53Ti47 alloy and provide dynamic lattice evolution information, neutron diffraction is conducted to the heated sample in real time. It is found that the metastable Ni4Ti3 phase is only obtained in the heat-treated alloy while the as-fabricated NiTi alloy produced by WAAM contains only NiTi and Ni3Ti. The generated Ni4Ti3 precipitates have increased the NiTi phase micro strains. The hcp-Ni3Ti lattice evolution is found inducing converse lattice shrinkage and expansion in bcc-NiTi lattice during the dissolution and precipitation of Ni3Ti, respectively. The WAAM induced residual stress in the as-fabricated alloy is tensile and during the residual stress relief, the thermal expansion rate of Ni and Ni3Ti are reduced. In addition, the thermal expansion coefficient of NiTi and Ni3Ti is measured according to the neutron Rietveld refinement as 0.040 × 10−3 °C−1 and 0.036 × 10−3 °C−1, respectively.