Carbon foam based on nature biomass resources has great prospective applications in water treatment, sensors, electronic devices, and electromagnetic shielding materials due to their low cost, stable surface properties, and excellent mechanical-electrical properties. Here, we reported a multifunctional carbon material with a 3D network structure from low cost and renewable cotton by a thermal decomposition method. Graphene oxide (GO) was adsorbed in cotton foam via a "dip-and-dry" method and thermal reduced to reduced graphene oxide (rGO) forming carbonized cotton/reduced graphene oxide (CC/rGO) foam. Compared with pure CC foam, the hydrophobicity and mechanical-electrical properties of CC/rGO foam were significantly enhanced because the foam structure was optimized by the introduction of rGO. The CC/rGO foam exhibited high oil-water separation capability (16-27 times of its own weight), mechanical stability (stable compression with 100,000 cycles), and satisfactory electromagnetic interference (EMI) shielding effectiveness (36 dB). These results illuminate a facile and sustainable strategy to modify and fabricate multifunctional biomass carbon foam.