Developing novel methods for oil/water separation is extremely important because of the increasing discharge of domestic oil pollution. For this purpose, raspberry-like polydopamine/SiO2 (PDA/SiO2) particles with a PDA core decorated by SiO2 is designed. A superhydrophobic surface originates from PDA/SiO2 that is covalently bonded to a natural cotton. Furthermore, a long alkyl-chain amine (octadecylamine) with low surface energy is grafted onto the hierarchical structure of composite cottons, which would make cotton have high water resistance and endow this composite with admirable superhydrophobicity. The modified composite cotton displays high hydrophobicity and oleophilicity, so it could effectively separate oil form oil/water mixtures. The separation efficiency of hexane/water mixture was as high as 97%. The composite cotton also has excellent recyclability, robust mechanical stability, chemical resistance, and pH stability.