Because of their low density, excellent dielectric properties, natural porous structure, environmental compatibility, and reproducibility, bio-carbon materials provide new ideas for the design of electromagnetic wave absorbing materials. Herein, the bio-carbon/FexOy composite absorbing materials have been successfully synthesized by a facile co-precipitation and then sintering method. The microwave absorbing properties of the materials can be adjusted by changing the type of the precursor biocarbon source and the sintering temperature. Among them, the lowest reflection loss of the Corn Leaf/Fe (CL/Fe) composite absorbent (900 °C) obtained using corn leaf as the precursor system is as high as − 41.1 dB at 14.56 GHz, with a thickness of 2.0 mm, and the absorption bandwidth below − 10 dB is up to 5.04 GHz (12.48–17.52 GHz). The electromagnetic wave absorption performance of the materials is analyzed in detail. It demonstrates the application prospects of bio-carbon materials in the field of microwave absorption.