Abstract: Magnetic functional biomass carbon (Fe₃O₄/porous biomass carbon) microwave absorbing materials with both dielectric loss and magnetic loss were prepared by carbonization and a hydrothermal method. With increasing FeCl₃ concentration, aggregated Fe₃O₄ nano-crystallites on the surface of the carbon increased under a hydrothermal temperature of 200℃, with a uniform particle size with an average of about 200 nm. The effects of the morphology, structure and Fe₃O₄ content on the microwave absorbing properties of the composites were investigated, and the mechanism of multi-loss microwave absorption was also analyzed. The results show that when the concentration of FeCl₃ is 4 mmol, the magnetically functionalized biomass-carbon composite has an excellent microwave absorbing performance in the range 2-18 GHz, and the thickness is 2 mm. The maximum reflectance loss at 11.42 GHz is up to -42.2 dB, and the effective bandwidth is 3.14 GHz (reflection loss is less than -10 dB).