Abstract: With the rapid development of precision guidance and radar detection technologies, radar absorbing materials have gained popularity. Traditional radar absorbing materials are limited because of a high density and narrow absorption band. To improve the absorption properties of traditional radar absorbing materials, developing radar absorbing material with thin-layer, light-weight, broadband and strong-absorbing is necessary. The effects of Co-doping on the structure, morphology, and microwave absorption properties of reduced graphene oxide (RGO)/Fe₃O₄ composites were studied in this paper. The RGO/Fe₃O₄ and Co-doped RGO/FeFe₃O₄ composites were prepared via a one-step hydrothermal method. The effects of Co on the microstructure, phase composition, and valence state of the composite were analyzed using scanning electron microscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy. The relative complex permittivity and permeability of RGO/Fe₃O₄ and Co-doped RGO/Fe₃O₄ composites were within 2-18 GHz, as recorded by a vector network analyzer. The influence of Co-doping on the microwave absorption property of RGO/Fe₃O₄ was simulated. The results show that a part of Co participates in the hydrothermal reaction to form CoCO₃, Co₃O₄, and Co₂O₃, whereas some Co exists in a simple form, and the Fe³⁺ coordination on the graphene oxide (GO) surface is affected through the positive and negative charge attraction mechanism, inducing Fe₃O₄ adhesion on the graphene surface. Co-doping improves the composites' electrical conductivity and magnetic loss ability thereby significantly enhancing their wave absorption property. Compared with RGO/Fe₃O₄, the maximum reflection loss of Co-doped RGO/Fe₃O₄ composites increases by 3.44 dB, and the effective absorption bandwidth of Co-doped RGO/Fe₃O₄ is broadened by 2.88 GHz when the matching thickness is 2.0 mm, whereas the maximum reflection loss increases by 8.45 dB, and the effective absorption band is broadened by 2.73 GHz when the matching thickness is 2.5 mm. The structure and morphology of RGO/Fe₃O₄ significantly changes by Co addition, which effectively improves the composites' absorption properties.