Abstract: The rapid expansion of military and civilian wireless technologies has exacerbated electromagnetic interference, creating a pressing need for high-performance low-frequency electromagnetic wave absorbers (EWAs). Herein, we report a self-assembled CuS (SA-CuS) architecture prepared via a dissolution–recrystallization mechanism. A high density of sulfur vacancies enhances local polarization, which effectively modulates the dielectric constant and boosts dielectric loss capability. These synergistic effects significantly strengthen the electromagnetic response of the material in the low-frequency range. The composite exhibits a minimum reflection loss (RL_min) of –66.9 dB, and across the C-band (4–8 GHz), it offers an effective absorption bandwidth (RL ≤ –10 dB) covering 42.5% of the entire C-band while maintaining an impedance-matching area ratio of 18.22%. This study demonstrates that rational microstructural engineering can precisely tailor electromagnetic properties, providing design principles and experimental basis for next-generation low-frequency EWAs in electromagnetic compatibility, stealth, and 5G communication systems.