The poor dispersity and oxidation resistance of ferromagnetic metal nanoparticles can induce serious deterioration in electromagnetic properties, which then significantly limits the application of this catalog of materials. In this work, sandwich-like Co/rGo/Co composites were in situ constructed, in which monodispersed Co nanoparticles with diameters of 20-60 nm were densely dispersed on both sides of rGO nanosheets. A connecting network between the densely-packed Co nanoparticles can be formed, where Co nanoparticles are abutted or bridged to each other through a neck of Co. These sandwich-like composites evidently contributed to improved permittivity and permeability, which was ascribed to the enhanced interface polarization and exchange coupling in this Co nanoparticles densely-packed structure. A maximum reflection loss (RL) of -61 dB (at 11.1 GHz) together with an efficient absorbing bandwidth (RL < -10 dB, ERL) of 4 GHz was obtained at a very thin matching thickness of 2 mm. The coating also presented a potential double-band absorbing performance, at SC band and Ku band, respectively. The excellent electromagnetic absorbing performances were ascribed to the synergistic effect of multiple dielectric losses and ferromagnetic losses. The sandwich-like Co/rGO/Co composites proposed an alternative way for broadband and high-efficiency absorption and provided a typical structure to analyze the loss mechanisms.