3D porous nanosheets composite comprised of graphitic carbon nitride (g-C3N4) and carbon was prepared by a simple pyrolysis and subsequent carbothermal activation method for the first time. The morphology and structural properties of obtained 3D porous g-C3N4/C nanosheets composite was systematically investigated. It was found that the morphology of the g-C3N4/C composite evolved from scroll-like structure to 3D porous nanosheets with ultrathin layer after 6 h activation, meanwhile, the specific surface area and pore volume were significantly increased. Such 3D porous nanosheets like these can function as effective catalysts for a range of gas-phase photocatalytic reactions such as photocatalytic reduction of CO2 with H2O vapor to produce value-added fuels. As a demonstration, outstanding photocatalytic activity was achieved on this 3D porous g-C3N4/C nanosheets composite, where ultra-high CO and CH4 yield of 229 and 112 urnol 1-cat, respectively, were acquired under 7 h simulated solar irradiation, which were 25-fold better than that of bulk g-C3N4. The excellent photocatalytic performance should be attributed to the advantageous structural and compositional features.