Porous materials with hierarchical structures are of increasing importance because of their potential application in separation and electronic technology. Herein, three-dimensional graphene foam (GF) was prepared by using polystyrene particles as a sacrificial template and an autoclaved leavening process, and the GF was further processed by carbon dioxide as an activation agent to produce meso- and nanopores in the foam. The resulting hierarchical graphene foam (HGF) exhibits high porosity, hydrophobicity and excellent thermal stability. Compared to regular GF, due to the integration of hierarchically porous structures, HGF manifests outstanding performance for oil adsorption when applied to separating oil-water mixtures, through a combination of hydrophobicity and capillary action.