Electromagnetic wave absorbing materials can dissipate energy by converting electromagnetic energy into thermal energy. Therefore, they are widely used in communication and military fields. Due to the environmental pollution and high costs associated with chemically synthetic composite materials,biomass⁃derived carbon materials have emerged as a prominent research focus.Given the intrinsic adsorption capacity of carbon, coconut shell biomass was selected as a precursor. A carbon/nickel composite absorbing material, designated as CE/Ni⁃x (where x denotes the immersion time in hours), was successfully synthesized via an in⁃situ growth and high⁃temperature reduction method with varying mass fractions of nickel oxide.The composite absorbing material was characterized and tested using an X⁃ray diffractometer, scanning electron microscope, and vector network analyzer.The results show that CE/Ni⁃7 exhibits absorption characteristics in both high⁃ and low⁃frequency bands, with a minimum reflection loss (RLmin) value of -30.05 dB.Furthermore, by adjusting the immersion time, effective absorption can be achieved in the 2.7-18.0 GHz frequency band (reflection loss lower than -10 dB). This research demonstrates a viable route for the low⁃cost, controllable synthesis of dual⁃band (low frequency/ high frequency) electromagnetic wave absorbing materials.