Abstract: Biomass derived carbon has attracted extensive attention in the field of microwave absorption due to its environmental protection and sustainability. In this study, Ni/phoenix tree oakum biomass derived carbon(Ni/PBDC)absorbent composite material was prepared by in-situ growth method using green impregnation carbonization process and biomass waste phoenix tree oakum as raw material. Research has shown that during the carbonization process of the phoenix tree oakum, the functional groups within the lignin undergo pyrolysis and transform into gases that are released, while this process is simultaneously accompanied by the formation of biochar and further graphitization. The growth of nickel particles on the surface and within the carbon enhances the interfacial polarization loss of the composite absorbing material. The addition of magnetic nickel particles increases both magnetic loss and interfacial polarization loss, but an excessively high nickel content can lead to impedance mismatch in the material, thereby reducing its ability to attenuate electromagnetic waves. The optimal Ni/PBDC composite absorbing material exhibits a minimum reflection loss of −40 dB, with an effective absorption bandwidth of 4.16 GHz at a corresponding matching thickness of 1.4 mm. It is worth noting that the response thickness of Ni/PBDC composite absorbing materials with the same nickel content is less than 1.5 mm. The composite absorbing material has the advantages of thin thickness, strong absorption performance and wide band, so it has great application potential in the field of wave absorption.