Abstract: The rapid advancement of modern electronic information technology and military stealth technology has led to increasing concerns regarding electromagnetic pollution and radar detection threats. Consequently, the development of high-efficiency, lightweight, and wide-band electromagnetic wave absorbing materials has emerged as a significant research focus within materials science. Absorbers derived from biomass materials not only leverage the unique morphological characteristics of these materials but also align with the principles of green and circular economic development. In this study, kapok fiber was utilized as the raw material to prepare CKF with a hollow structure through direct carbonization. The effects of carbonization temperature and absorber concentration on electromagnetic parameters and absorption performance were systematically investigated. By comparing the hollow structure of CKF with the solid structure of pulp-based carbon fiber, it was demonstrated that the hollow structure enhances electromagnetic wave attenuation through multiple reflections and increases the likelihood of electromagnetic wave interference loss. When the CKF filling ratio was 7% (by mass) and the material thickness was 2.5 mm, the effective absorption frequency band (reflection loss less than −10 dB) reached 5.2 GHz, with the lowest reflection loss (RL_min) achieving −33.7 dB. These results indicate superior absorption performance compared to solid carbon fiber derived from pulp.