The paper aims to adjust the electromagnetic matching of Co3O4 nanoparticles to achieve the best electromagnetic wave absorption performance, and obtain electromagnetic wave absorbing materials of lightweight, strong absorption, broadband and small thickness. The biomass-rGO was prepared from corn stover, and then the biomass-rGO was introduced into the Co3O4 nanoparticles by the hydrothermal method to prepare a Co3O4/biomass-rGO nanosheet of heterostructure. The composition, morphology and structure of Co3O4/biomass-rGO heterostructure nanosheets were characterized by X-ray diffraction analyzer, transmission electron microscopy and high resolution transmission electron microscopy. At the same time, the influence of introducing biomass-rGO on the absorbing properties of Co3O4/biomass-rGO heterostructure nanosheets was analyzed by a vector network analyzer. The introduction of biomass-rGO significantly improved the electromagnetic wave absorption performance of Co3O4/biomass-rGO heterostructure nanosheets in the frequency range of 2~18 GHz, which not only reduced the effective absorber thickness, but also expanded the effective absorption frequency bandwidth. A maximum reflection loss value of –36.1 dB was achieved at a thickness of 1.5 mm and a frequency of 15.8 GHz. The effective absorption frequency bandwidth was 15 GHz, and there was effective absorption in the S, C, X, and Ku bands, and all effective absorption in a wide thickness range of 1.0 to 5.5 mm was achieved. It turns out that the introduction of biomass-rGO can effectively improve the electromagnetic matching and dielectric loss of Co3O4.