The increasing concern over the negative impact of electromagnetic radiation and interference on humans has led to a growing interest in microwave-absorbing materials that are cost-effective, have a wide frequency range, and have high efficiency. In this paper, an FeO nanoparticle/carbonized hemp fiber composite was successfully prepared using hemp fibers as the primary material and template. By carefully regulating the concentration of the iron nitrate impregnation solution, accurate loading of FeO nanoparticles onto the carbonized hemp fiber was achieved. Due to its unique porous structure, the balance between impedance matching, and electromagnetic loss, the prepared FeO nanoparticle/carbonized hemp fiber composite exhibits light weight, high absorption strength, and broadband absorption characteristics. The broadest absorption bandwidth of 6.1 GHz can be achieved, covering the entire Ku-band, and the minimum refection loss is as low as -49.7 dB. More interestingly, the Fe3O4 nanoparticle/carbonized hemp fiber composite exhibits attractive microwave absorption performance in both the X-band and Ku-band even with a wide range of FeO nanoparticle loading. Furthermore, simulations of the radar cross section (RCS) have confirmed that the FeO nanoparticle/carbonized hemp fiber composite is effective in attenuating electromagnetic waves in a real environment. This work presents an economical and efficient method for the development of porous carbon-based absorbents.