Functionally graded material has been increasingly attractive due to the locally designed and optimized material properties. In the present paper, to achieve high thermal lattice structural stability and high corrosion resistance in each side of one bulk material, an Fe3Ni–FeNi compositionally graded material (FGM) bulk sample has been fabricated using an innovative wire-arc additive manufacturing process. A subsequent homogenization post-production heat treatment is applied to stabilize the phase composition and eliminate the possible segregation in the as-fabricated sample. Meanwhile, the high intensity neutron diffraction instrument WOMBAT is conducted to measure the phase evolution processes in the bulk sample in-situ, thus the thermal induced lattice evolutions of Fe3Ni and FeNi can be accurately obtained dynamically in the function of time. According to the results, the WAAM has successfully fabricated the target FGM with uniform Ni content gradient. However, hot cracking defects are found in the bulk sample because of the inadequate interpass temperature setting. As analyzed from neutron diffraction results, the homogenization heat treatment has improved the phase thermal stability of the FGM and the application temperature of the Fe3Ni–FeNi FGM should be limited under 630 °C.