•A unique g-C3N4/Fe3CNC/Co-NC heterostructure is prepared by simple mixing and two-step pyrolysis.•After the structural and phase reconfiguration, small Fe3C@NC and Co@NC core-shell nanoparticles are anchored on porous g-C3N4 bridges.•The g-C3N4/Fe3CNC/Co-NC heterostructure delivers remarkable electromagnetic wave absorption. Developing electromagnetic wave (EMW) absorbers with low-cost, lightweight, strong absorption capacity, thin matching thickness, and wide effective absorption bandwidth (EAB) is still a significant challenge. Herein, we report a unique g-C3N4/Fe3CNC/Co-NC heterostructure by simple mixing and two-step pyrolysis of melamine, Fe3O4, and ZIF-67. After the structural and phase reconfiguration, N-doped carbon layer-coated Fe3C nanoparticles (NPs) assembled nanoshells (Fe3C@NC) and N-doped graphitic carbon-coated metallic Co NPs (Co@NC) are formed, and the core-shell NPs are interconnected by porous g-C3N4 bridges. Profiting from the successful construction of the unique g-C3N4/Fe3CNC/Co-NC heterostructure and the synergetic effect of multi-component, multiple core-shell structure, and strong interaction, the as-fabricated g-C3N4/Fe3CNC/Co-NC inherits the excellent EMW absorption performance with a RL value of -25.4 dB at a thin matching thickness of 2.6 mm. The EAB can reach up to 8.64 GHz at a thickness of 2∼5 mm. This study provides the contribution of structural and phase reorganization to the field of EMW absorption. Display omitted