The problems of electromagnetic wave (EMW) pollution in X and Ku bands (8–18 GHz) are becoming more and more serious. Therefore, it is urgent to design EMW absorbing materials with high-efficiency such as thin thickness, lightweight, wide bandwidth and strong EMW absorption. Inspired by the biomorph of sea cucumber, Nb₂CT_x MXene@Co nitrogen-doped carbon nanosheet arrays@carbon fiber aerogels (Nb₂CT_x@Co-NC@CFA, Nb₂CT_x = niobium carbide) were constructed by self-assembly, in-situ chemical deposition and subsequent pyrolysis. The carbon fiber aerogel, as the basic skeleton of sea cucumber, forms lightweight three-dimensional interconnected conductive network, enhances the dielectric loss and extends the multiple reflection and absorption paths of EMW. As the tentacles of sea cucumber surface, Nb₂CT_x MXene and Co nitrogen-doped carbon nanosheet arrays exist rich heterogeneous interfaces, which play an important role in improving EMW polarization loss and optimizing impedance matching. The minimum reflection loss (RL_min) of Nb₂CT_x@Co-NC@CFA reaches –54.7 dB at 9.84 GHz (2.36 mm) with a low filling ratio of 10 wt.% and the effective absorption bandwidth (EAB) of Nb₂CT_x@Co-NC@CFA reaches 2.96 GHz (8.48–11.44 GHz) with 2.36 mm and 5.2 GHz (12.8–18 GHz) with 1.6 mm, covering most of X and Ku bands by adjusting thickness. The radar cross section (RCS) value of Nb₂CT_x@Co-NC@CFA is 26.64 dB·m², which is lower than that of the perfect electrical conductor (PEC), indicating that Nb₂CT_x@Co-NC@CFA can effectively decrease the probability of the target being detected by the radar detector. This work provides ideas for design and development of EMW absorbing materials with high-efficiency EMW absorption in X and Ku bands.