Polymeric graphitic carbon nitride (g-C3N4) materials are promising catalysts to compete with precious metals, but they mostly only show moderate catalytic activity, particularly low oxidation power for oxygen evolution reaction (OER). Although activating strategies, such as nanostructure designing and band structure engineering, have led to remarkable activity gains, there remains considerable room to improve performance that aims for real applications. We report here a novel strategy-protonating supramolecular aggregates followed by thermal polycondensation-to prepare porous g-C3N4 nanosheets (SH-g-C3N4) with high oxidation power, which employ as bifunctional OER catalyst and highly efficient photocatalyst material. The OER onset potentials of 1.47 V and Tafel slope of 128 mV dec(-1) over SH-g-C3N4 are superior to the g-C3N4 obtained from supramolecular aggregates without protonation (S-g-C3N4). Furthermore, its Tafel slope would be reduced to 63.6 mV dec(-1) under visible light irradiation, which is comparable to that of the active commercial RuO2 and IrO2 catalysts. Interestingly, the achieved nanosheets display enhanced visible light photocatalytic activity. The striking catalysis performance of SH-g-C3N4 can be ascribed to the unique advantage of its highly porous structure and favorable electronic structure and the generation of lots of center dot O-2(-) and center dot OH radicals promoting the kinetic process. Our theoretical calculations reveal the mechanism of photoelectric catalyst for oxygen evolution reaction. This work provides a facile and scalable method to design highly efficient polymeric catalysts.