Polymer membranes with switchable wettability have promising applications in smart separation. Hereby, we report highly porous poly(styrene-co-N,N-(diethylamino)ethyl methacrylate) (i.e., poly(St-co-DEA)) membranes with "open-cell structure and CO2-switchable wettability prepared from water-in-oil (W/O) high internal phase emulsion (HIPE) templates. The open-cell porous structure facilitates fluid penetration through the membranes. The combination of CO2-switchable functionality and porous microstructure enable the membrane with CO2-switchable wettability from hydrophobic or superoleophilic to hydrophilic or superoleophobic through CO2 treatment in an aqueous system. This type of membrane can be used for gravity-driven CO2-controlled oil/water separation, in which oil selectively penetrates through the membrane and separates from water. After being treated with CO2 switching wettability of the membrane, a reversed separation of water and oil can be achieved. Such a wettability switch is fully reversible, and the membrane could be regenerated through simple removal of CO2 and oil residual through drying. This facile and cost-effective approach represents the development of the first CO2-switchable polyHIPE system, which is promising for smart separation in a large volume.