Composites of carbon and magnetic metal are always the most attractive candidates for high-performance microwave absorbing materials (MAMs) due to their synergetic loss mechanisms and tunable electromagnetic properties. Herein, FeCo alloy nanoparticles have been innovatively coupled with ordered mesoporous carbon through impregnation and in situ hydrogenthermal reduction. Incorporation of FeCo alloy nanoparticles into mesoporous carbon provides a significant enhancement in microwave absorption, and both strong reflection loss (-73.8 dB at 10.6 GHz) and wide response bandwidth (4.0-18.0 GHz over -10.0 dB) can be simultaneously achieved in the optimized composite. This improved microwave absorption property is much better than that of many mesoporous carbon-based composites ever reported. Electromagnetic analysis reveals that FeCo alloy nanoparticles display dual electromagnetic functions. On the one hand, they produce obvious magnetic loss ability in the composites, and on the other hand, they moderately weaken the dielectric loss ability as compared to pristine mesoporous carbon. A delta function calculation indicates that the narrowed gap between complex permittivity and complex permeability is beneficial to creating the matched characteristic impedance, which affords the prerequisite for the formation of desirable microwave absorption performance. This study not only identifies FeCo alloy/mesoporous carbon as promising MAMs and reveals the origin of the microwave absorption enhancement, but also paves a new way for designing magnetic carbon-based MAMs in the future.