The Co/C nanocomposites toward outstanding electromagnetic (EM) microwave absorption were meticulously designed and prepared by bio-gel method. The Co nanoparticles were embedded into the carbon network constructed by agar powder. Our obtained Co/C nanocomposites exhibited excellent EM wave absorption performance including wide bandwidth (7.1 GHz), thin thickness (1.71 mm), light weight and wide effective absorbing frequency. Dielectric relaxation at multilayer interfaces, porous biomass carbon materials and magnetic Co nanoparticles are the main sources for enhancing the wave absorption properties. The amorphous carbon in the material prevents the condensation and oxidation of magnetic Co nanoparticles, thus contributing to the generation of good impedance matching. On the basis of the percolation theory, the optimal Co content was determined, and a percolation model suitable for Co/C materials was established.