Composites composed of ferromagnetic alloy/dielectric material have been widely studied and applied. The alloying of the two metals endows these composites with escalated magnetic loss capacity, but they are usually micro-sized, which is not conducive to further improving the absorption performance. Herein, we develop an efficient approach to synthesize Co7Fe3@C nanocomposites with particle sizes ranging from 32 to 42 nm based on the in-situ carbon coating on the FeCo2O4 precursor and subsequent carbothermal reduction procedure. The thickness of carbon coating can be effectively tailored by controlling the reaction conditions. The minimum reflection loss (RL) value of the Co7Fe3@C nanocomposite with a carbon coating of 2.5 nm goes up to ?100.8 dB at only 1.7 mm thickness, and the effective absorption bandwidth (EAB) also reaches 9.9 GHz (8.1–18 GHz) at 2.0 mm. Moreover, as the carbon coating increases to 4.2 nm, the bandwidth escalates to 11.5 GHz (6.5–18 GHz) at 2.0 mm, which covers the whole X and Ku bands and half of the C band. The remarkable performance originates from the strong magnetic loss capability of the nano-sized Co7Fe3 core as well as the good impedance matching brought by appropriate carbon coating. Accordingly, the Co7Fe3@C nanocomposite is convinced to be a competent candidate among absorbers with high absorption intensity and ultra-wide broadband.