In this work, we report a simple, facile and effective method to simultaneously hydrogenate and exfoliate graphitic-C₃N₄ (g-C₃N₄) through high concentration sulfuric acid treatment. The hydrogenation mechanism of g-C₃N₄ is explained experimentally and it is further revealed in detail by molecular structure dynamics as well as the corresponding electronic structure evolutions. Five different atomic sites in unit cell of g-C₃N₄ are structurally available to be hydrogenated, and four of them are energetically favored to form hydrogenated structures. Different from the pristine g-C₃N₄ that is flat in basal plane, the energetically favored hydrogenation structure of g-C₃N₄ possesses the corrugated fluctuation plane. The hydrogenated g-C₃N₄ structures also present blueshifted UV–vis absorption and photoluminesce (PL) peaks compared to that of pristine g-C₃N₄, and it is well explained by theoretical calculation results that the bandgap becomes larger due to hydrogenation. Finally, it is found that the photocatalytic performance of g-C₃N₄ is dramatically enhanced once the crystal structure is hydrogenated. The enhanced photocatalytic performance is mainly attributed to the hydrogenation caused spatial charge separation due to the redistribution of charge density in both valence band maximum and conduction band minimum. The revealing of spatial charge separation provides insight into the deep understanding of hydrogenation mechanism of g-C₃N₄, which is critically significant for designing light-efficient photocatalysis.