Increasingly serious electromagnetic pollution problems bring the threaten for national defense security and personal health, biomass carbon materials derived absorbers have arouse widespread interest due to their green, easy accessibility, structural diversity, rich functional groups. Herein, biomass kelp derived three-dimension (3D) porous carbon/Fe3O4 (KPC/Fe3O4) nanocomposites were successfully synthesized vis KOH activation as well as in-situ growth of Fe3O4 and following pyrolysis. Attributed to multiple transmission paths for electromagnetic wave from 3D porous structure, dielectric-magnetic synergistic effect, and numerous defects in the kelp-derived carbon with polarization loss caused by heteroatom doping and interfacial polarization, KPC/Fe3O4 nanocomposites possess outstanding impedance matching and extraordinary attenuation ability. In particular, the minimum reflection loss (RLmin) value of KPC/Fe3O4-0.1 nanocomposite reaches −75.02 dB at 10.5 GHz as well as the effective absorption bandwidth (EAB, RL < -10 dB) value achieves 4.83 GHz with a thickness of 2.85 mm. Unique structures and excellent EWA performance demonstrate that KPC/Fe3O4 nanocomposites can serve as potential absorbers for military and civilian fields.