Abstract: With the growing concerns over electromagnetic pollution, low-cost, environmentally friendly, and high-performance electromagnetic wave absorption (EWA) materials have attracted significant attention. This study reports the synthesis of porous Fe3O4/C composites, which incorporate both dielectric and magnetic loss mechanisms, through the carbothermal reduction method and the optimization of waste ratio to enhance EWA performance. The Fe3O4/C-10 composites, with 10 wt% soybean residues, demonstrated the best EWA performance, achieving a minimum reflection loss of -56.4 dB and a bandwidth of 2.14 GHz at a thickness of 2.23 mm. This enhanced EWA performance was primarily attributable to the improved impedance matching and the synergistic effect between dielectric and magnetic losses. Furthermore, radar cross-section simulations confirmed the practical feasibility of the porous Fe3O4/C composites. This study proposes a viable strategy for utilizing soybean residue and electrolytic manganese residue, highlighting their potential application in EWA.