Abstract: Biomass materials have specific microscopic morphology and pore structure, which can be considered as excellent morphogenetic materials for carbon components. In addition, biomass materials are widely used, inexpensive and it is in line with the strategic needs of national sustainable development. In the research, we selects biomass cattail as the main carbon source and Fe³⁺ as the metal source. Fe/C composites can be obtained by in situ adsorption and carbon thermal reduction, where Fe nanoparticles are uniformly loaded on the surface of carbon substrate. Moreover, the crystallinity of Fe nanoparticles can be enhanced with the increase of calcination temperature. Fe/C-700 composites have multiple resonance behaviors at low and high frequencies, which contribute to the enhancement of dielectric loss capability. Two-dimensional reflection loss results show that Fe/C-900 composites exhibit the excellent microwave absorption performance, where the minimum reflection loss can reach −35 dB at 4.4 GHz with the thickness of 5 mm. The superior microwave absorption performance can be attributed to its better impedance matching characteristics and the synergistic effect of dielectric loss and magnetic loss, and the study will provide a new strategy for biomass-derived magnetic carbon-based microwave absorption composites.