The work aims to prepare the CB/RGO composite coating with excellent microwave absorbing properties. Reduced graphene oxide (RGO) was in-situ grown into the conductive carbon black (CB) system for synthesizing the CB/RGO composite absorbent by the liquid phase method, and the CB/RGO composite coatings were prepared by epoxy resin as matrix. The microstructure of the prepared CB/RGO composite absorbent was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effect of the amount and the thickness of absorbent on the electromagnetic properties of the coatings was studied. From the microstructure analysis, the CB particles adhered to the graphene sheets with a “grape-like” structure, and achieved coating growth on the surface, which were dispersed evenly and had good adhesion. The prepared CB/RGO composite coatings possessed a uniform thickness and a density of only 1.1 g/cm3, and had both the lightweight and flexible characteristics. The microwave reflectance test results indicated that the coatings had no obvious strong electromagnetic absorption within high filler loadings of 3.0% and 3.7%, while the coatings showed excellent microwave absorption properties within low filler loadings of 1.6% and 2.3%. When the filler loading is 2.3% and the thickness is 1.9 mm, the coating exhibits the best absorbing performance, the maximum absorbing intensity is -17.1 dB, and the effective absorbing bandwidth reaches 6.63 GHz covering 66.3% of the entire measured bandwidth, showing good broadband absorbing performance. In addition, when the thickness is 2.5 mm, the coating with a filler loading of 2.3% realizes full absorption of the radar wave in the X-band.