Abstract: In the field of broadband metamaterial absorbers, most research efforts have focused on optimizing the resonant layers and designing multi-layer structures, but relatively little attention has been paid to the dielectric layers themselves. This paper proposed a method using carbonyl iron powder to modify the dielectric layer. This method significantly enhances the electromagnetic wave attenuation capability of the dielectric layer with the X-band range for metamaterial absorbers. A broadband absorber with a reflection loss (RL) of less than –10 dB within the frequency range of 4.98–18 GHz and covering the C, X, and Ku band was designed. This work analyzed the surface current distribution and the power loss distribution to elucidate the absorption mechanism of the absorber. It was found that the modified dielectric layer accounted for more than 30% of the total loss in the 2–18 GHz frequency band, and the effective absorption bandwidth (RL ≤ –10 dB) was almost twice that of the unmodified dielectric layer. This enhancement in absorption bandwidth is attributed to the introduction of a new electromagnetic wave loss mechanism by carbonyl iron powder. Meanwhile, the absorber exhibited good angular stability, maintaining at least 80% absorption (RL ≤ –7 dB) in the 7.0–18.0 GHz range even when the incident angle was increased to 60°. The experimental results showed that the measured results matched the simulation results well. Furthermore, compared with other methods for broadening the absorption bandwidth, the metamaterial absorber obtained by this method offers several advantages, including wideband absorption, thin profile, and a simple manufacturing process. This approach provides a new and promising direction for the design of broadband absorbers.