In recent years, biochar materials have attracted considerable attention in the field of electromagnetic wave absorption. This is due to the fact that they are a renewable resource, inexpensive and environmentally friendly. However, there is still a need to optimize their performance. Subsequently, the asymmetric diffusion at the alloy/oxide interface was induced by heat treatment and Kirkendall effect, and hollow magnetic particles were in situ generated on the porous substrate of biochar. Using biochar with a pyrolysis temperature of 800 °C, the obtained CoFe@CoFe 2O 4/C-800 composite exhibits excellent wave absorption properties: an optimum reflection loss of -71.4 dB is obtained at a thickness of 2.70 mm; when the thickness is reduced to 1.70 mm, the effective absorption bandwidth extends to 5.68 GHz, covering a large part of the Ku band. Mechanistic studies show that the excellent broadband absorption properties of the material are due to the unique hollow heterostructure. Through the dielectric-magnetic double loss synergy, multi-stage polarization effect and optimized impedance matching mechanism, the efficient dissipation of electromagnetic energy is realized. This work offers a novel concept for the development of eco-friendly, high-performance electromagnetic wave-absorbing materials.