Abstract: Microwave absorbers have great potential for military and civil applications. Herein, Co_0.5Zn_0.5Fe₂O₄/residual carbon (CZFO/RC) composites have been successfully prepared using a hydrothermal method. RC was derived from coal gasification fine slag (CGFS) via pickling, which removes inorganic compounds. Multiple test means have been used to study the chemical composition, crystal structure, and micromorphology of the CZFO/RC composites, as well as their electromagnetic parameters and microwave absorption (MA) properties. The CZFO/RC composites exhibit excellent MA performance owing to their dielectric and magnetic losses. When the thickness of CZFO/RC-2 (FeCl₃·6H₂O of 0.007 mol, ZnCl₂ of 0.00175 mol, and CoCl₂·6H₂O of 0.00175 mol) is 1.20 mm, the minimum reflection loss (RL_min) is −56.24 dB, whereas at a thickness of 3.00 mm and 6.34 GHz, RL_min is −45.96 dB and the maximum effective absorption bandwidth is 1.83 GHz (5.53–7.36 GHz). Dielectric loss includes interface and dipole polarizations, while magnetic loss includes current and remnant magnetic loss. CZFO/RC-2 exhibits high impedance matching, allowing microwave to enter the absorber. The computer simulation technology confirms that CZFO/RC-2 considerably decreases the radar cross-section. This study can be used to promote the use of CGFS as electromagnetic wave (EMW)-absorbing materials.