Abstract: With growing concerns regarding electromagnetic pollution, low-cost, environmentally friendly, and high-performance electromagnetic wave absorption (EWA) materials have attracted significant attention. This paper reports on the synthesis of porous Fe₃O₄/C composites that incorporate dielectric and magnetic loss mechanisms via the carbothermal reduction method and optimization of waste ratio to enhance EWA performance. The Fe₃O₄/C composites with 10wt% soybean residues (Fe₃O₄/C-10), demonstrated the best EWA performance, achieving the 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 is primarily attributable to improved impedance matching and the synergistic effect between dielectric and magnetic losses. Furthermore, radar cross-sectional 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 applications in EWA.