Carbon micro/nanocoils with excellent electromagnetic cross-polarization capability have promising applications in the microwave absorption field. However, carbon micro/nanocoils with hierarchical porous structures remain challenging. Herein, a novel catalyst-free strategy and a one-step carbonization process were provided to fabricate biomass-driven carbon microcoils with hierarchical porous structures from waste biomass. Due to the synergistic effects of interfacial polarization, multiscale relaxation, and negative imaginary permeability formed by the helical/chiral open-loop pore structure, the porous carbon microcoils (PCMC) exhibits optimized impedance matching, improved dielectric loss, and enhanced electromagnetic wave attenuation and absorption efficiency. The minimum reflection loss (RLmin) of PCMC reaches −63.65 dB at 4.68 GHz, and the RLmin of PCMC reaches −27.6 dB even at 1.59 mm thickness. The maximum effective absorption bandwidth (EAB) can reach 5.78 GHz at 1.86 mm matching thickness. These results can serve as a basis for reusing other green wastes and for manufacturing advanced PCMC-based microwave absorbers at low cost.