<p>FeNi36 is employed in aerospace, remote sensing, and mapping applications due to its low coefficient of thermal expansion, making it suitable for high-precision micro-parts. Customizing large FeNi36 parts through wire arc additive manufacturing has emerged as a new research direction. However, the high temperature gradient and cooling rate during the wire arc additive manufacturing process result in a coarse and uneven microstructure, leading to significant anisotropy in the mechanical properties and thermal expansion coefficient of FeNi36, which does not meet high-precision performance requirements. This study presents the development of a macro–micro volume fluid phase field model to simulate heat transfer and dendrite growth during wire arc additive manufacturing FeNi36 thin-walled specimens. The simulation results closely match the experimental data. By comparing the simulation results with the experimental data, the microstructure of wire arc additive manufacturing FeNi36 was optimized, and more suitable process parameters were identified. The mechanical property anisotropy of FeNi36 specimens prepared with these parameters is minimal. The tensile strength of specimens in various directions and positions ranges from 545 to 575&#xa0;MPa, with elongation between 26.8 and 30.2%. Furthermore, the thermal expansion coefficient curves of the samples in different directions are nearly identical, all being below 2.0 × 10⁻⁶ K⁻¹, which satisfies the commercial FeNi36 requirements.</p>