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Fabrication of electromagnetic wave-absorbing materials derived from biomass have aroused great attention due to their special structures, eco-friendly and economical qualities. Herein, ultralight hierarchical porous carbon (UHPC) is successfully prepared through simple swelling-freeze drying and pyrolysis. UHPC possesses hierarchical porous architecture, which is inherited and expanded from but greater than the inherent unique macro/microporous structures of tremella. The hierarchical porous architecture derived from tremella provides a long conductive path and more interfaces to form conductance loss and polarization loss. It causes multiple reflections and scattering of electromagnetic waves, as well. The swelling process of tremella can offer dipole polarization and defect polarization. As a bio-derived microwave-absorbing carbon material, it holds good impedance matching and high attenuation constant. Particularly, the minimum reflection loss (RL) value of UHPC750-3 (obtained at 750 °C) can be reached at −48.57 dB with a small matching thickness of 1.7 mm. Remarkably, the effective absorption bandwidth is up to 4.55 GHz at 2.5 mm. The results show that absorbing properties of UHPC can be adjusted by changing pyrolysis temperature and swelling rate. The corresponding absorption mechanism is discussed as well. This method provides an effective strategy to prepare biomass-derived microwave absorbents which show great potential for practical applications. • The inherent hierarchical and nanopore structures of tremella can be retain using the freeze-drying method. • The regulation of microstructure of porous carbon materials could be synchronously fulfilled by controlling swelling rate. • The performance with RL min of −48.57 dB and effective absorption bandwidth of 4.55 GHz were achieved at only 1.7 mm. [ABSTRACT FROM AUTHOR]