Abstract: Under the background of a transformation of the global energy structure, coal gasification technology has a wide application prospect, but its by-product, the coal gasification residue (CGR), is still not being efficiently utilized for recycling. The CGR contains abundant carbon components, which could be applied to the microwave absorption field as the carbon matrix. In this study, Fe/CGR composites are fabricated via a two-step method, including the impregnation of Fe³⁺ and the reduction process. The influence of the different loading capacities of the Fe component on the morphology and electromagnetic properties is studied. Moreover, the loading content of Fe and the surface morphology of the Fe/CGR can be reasonably controlled by adjusting the concentration of the ferric nitrate solution. Meanwhile, Fe particles are evenly inserted on the CGR framework, which expands the Fe/CGR interfaces to enhance interfacial polarization, thus further improving the microwave-absorbing (MA) properties of composites. Particularly, as the Fe³⁺ concentration is 1.0 mol/L, the Fe/CGR composite exhibits outstanding performance. The reflection loss reaches −39.3 dB at 2.5 mm, and the absorption bandwidth covers 4.1 GHz at 1.5 mm. In this study, facile processability, resource recycling, appropriately matched impedance, and excellent MA performance are achieved. Finally, the Fe/CGR composites not only enhance the recycling of the CGR but also pioneer a new path for the synthesis of excellent absorbents.