The performance of Fe₃O₄-based electromagnetic wave-absorbing materials is typically hindered by their low conductivity. Therefore, the introduction of carbon components with a rationally constructed microstructure has evolved as an effective approach for enhancing the electromagnetic properties of metal-oxide-based microwave absorbers. In this study, yolk–shell Fe–Fe₃O₄@C nanoparticles were synthesized via a hydrothermal–polymerization–vacuum carbonization method under a N₂ flow. In addition to a polymer with suitable layer thickness, which is required to obtain a mixed Fe and Fe₃O₄ phase, an appropriate carbon layer is needed. Furthermore, Fe@C with cavities can be fabricated when the thickness of the polymer layer is higher than the optimum value. The optimized reflection loss, effective bandwidth, and thickness of Fe–Fe₃O₄@C were −51 dB, 5.1 GHz (12.9–18 GHz), and 1.2 mm, respectively, which were larger than those of most Fe₃O₄-based absorbing materials reported to date. The excellent microwave absorption performance of Fe–Fe₃O₄@C was attributed to its excellent electromagnetic properties, including complex permeability and permittivity, and yolk–shell structure, which favored multiple reflections and scatterings and multiple polarizations at the core–cavity and cavity–shell interfaces.