Abstract: The rapid development of 5G communication technology and smart electronic and electrical equipment will inevitably lead to electromagnetic radiation pollution. Enriching heterointerface polarization relaxation through nanostructure design and interface modification has proven to be an effective strategy to obtain efficient electromagnetic wave absorption. Here, this work implements an innovative method that combines biomimetic honeycomb superstructure to constrain hierarchical porous heterostructure composed of Co/CoO nanoparticles to improve the interfacial polarization intensity. The method effectively controlled the absorption efficiency of Co²⁺ through delignification modification of bamboo, and combined with the bionic carbon-based natural hierarchical porous structure to achieve uniform dispersion of nanoparticles, which is conducive to the in-depth construction of heterogeneous interfaces. In addition, the multiphase structure brought about by high-temperature pyrolysis provides the best dielectric loss and impedance matching for the material. Therefore, the obtained bamboo-based Co/CoO multiphase composite showed excellent electromagnetic wave absorption performance, achieving excellent reflection loss (RL) of –79 dB and effective absorption band width of 4.12 GHz (6.84–10.96 GHz) at low load of 15wt%. Among them, the material’s optimal radar cross-section (RCS) reduction value can reach 31.9 dB·m². This work provides a new approach to the micro-control and comprehensive optimization of macro-design of microwave absorbers, and offers new ideas for the high-value utilization of biomass materials.