Tuning photochemistry conversion efficiency by atomic-level tailoring will unlock great potential for pursuing higher photocatalytic performance for graphitic carbon nitride (g-C3N4). Here, a novel strategy to fabricate amorphous carbon���engineered ultrathin g-C3N4 nanocomposites, endowing the engineered g-C3N4 with a much higher H2 evolution rate, reaching an optimum value as high as 746.95 ��mol h���1 g���1, 15.4 times higher than that of bulk g-C3N4, is described. Interestingly, with the formation of intimate interfaces between amorphous carbon and ultrathin g-C3N4, the interfacial charge transfer is boosted significantly and the recombination rate of photogenerated electrons and holes could be highly reduced, thus leading to a higher quantum yield. Moreover, the thickness of the g-C3N4 is significantly reduced by the steric-hindrance effect of amorphous carbon grown in situ, and the as-prepared ultrathin g-C3N4 shows a suppressed intersystem crossing rate in the photocatalytic H2 evolution process, thus leading to a lower triplet exciton concentration in the energy conversion process, and also faint triplet���triplet annihilation. It is believed that the present work identifies a new pathway to understanding the role of carbon in nanostructure construction, and will be of broad interest in research on engineering metal-free carbon-based catalysts and on solar conversion systems.