Abstract: Electromagnetic interference, which necessitates the rapid advancement of substances with exceptional capabilities for absorbing electromagnetic waves, is of urgent concern in contemporary society. In this work, CoFe₂O₄/residual carbon from coal gasification fine slag (CFO/RC) composites were created using a novel hydrothermal method. Various mechanisms for microwave absorption, including conductive loss, natural resonance, interfacial dipole polarization, and magnetic flux loss, are involved in these composites. Consequently, compared with pure residual carbon materials, this composite offers superior capabilities in microwave absorption. At 7.76 GHz, the CFO/RC-2 composite achieves an impressive minimum reflection loss (RL_min) of −43.99 dB with a thickness of 2.44 mm. Moreover, CFO/RC-3 demonstrates an effective absorption bandwidth (EAB) of up to 4.16 GHz, accompanied by a thickness of 1.18 mm. This study revealed the remarkable capability of the composite to diminish electromagnetic waves, providing a new generation method for microwave absorbing materials of superior quality.