FeNi intermetallic has been continuously attractive due to the appreciable magneto-crystalline anisotropy and high saturation magnetization in the nominally-equiatomic chemically ordered L10 structure. In the present research, the bulk FeNi alloy is successfully fabricated using an innovative wire-arca additive manufacturing process, which is also an in-situ alloying method. In the meantime, to characterize the thermal lattice evolution of the FeNi, neutron diffraction is conducted to the as-fabricated sample in-situ during a heat treatment process. According to the neutron results, an anisotropic fcc-FeNi lattice evolution has been detected, which indicates the existence of FeNi superstructure in the WAAM fabricated FeNi bulk material. In addition, due to the featured advantage of the neutron data, the fcc-FeNi volumetric lattice thermal expansion coefficients during heating and cooling processes are calculated.