High performance broadband microwave absorption material is in urgent demand as the radar detection frequency range has been greatly broadened and electromagnetic radiation pollution has become increasingly serious. In this work, we fabricated a hierarchical bowknot-like Co/CoO@C nanocomposite with a two-step hydrothermal method, followed by high-temperature pyrolysis under argon atmosphere, which is composed of amorphous carbon and uniformly dispersed irregular Co/CoO nanoparticles. Complex permittivity and complex permeability were modulated by varying the calcination temperature (500-700 degrees C). Reflection loss (RL) curves of the Co/CoO@C nanocomposites show superior broadband electromagnetic wave absorption performance. Especially, the effective absorption bandwidth was maximized to 13.6 GHz (4.4-18 GHz) at the thickness of 6 mm, and a - 45.0 dB minimum RL appears at 6 GHz. Such superior broadband electromagnetic wave absorption properties are attributed to the synergistic effect of the hierarchical bowknot-like structure and strong surface/interface polarization, as well as the eddy current and magnetic natural resonance. These results indicate the bowknot-like Co/CoO@C nanocomposite is promising in dealing with broadband electromagnetic radiation pollution and broadband military radar detection.