Graphitic carbon nitride (g-C3N4) is one of the cardinal semiconductor photocatalysts to address the energy crisis issues. Howbeit, it remains a large challenge to explore effective strategies to confront its flaws, such as insufficient response to solar spectrum and less efficient charge separation. Here, we report development of a widerange-photoresponsive O substituted g-C3N4 with tunable band structure and efficient charge separation by an one-pot co-pyrolysis of melamine and substantial ammonium acetate, and the O substitution concentration can be largely adjusted from 0.68% to 5.59% by controlling the ammonium acetate amount. Experimental and theoretical results uncovered that the O atoms were incorporated into the g-C3N4 lattice by replacing the C-N=C coordinated N atoms to form C-O=C bonds. This atomic substitution introduces an acceptor level below conduction band of g-C3N4, which not only broadens visible-light response of g-C3N4 to 800 nm with adjustable band gap, but also greatly promotes the carrier density, bulk charge separation and surface charge transfer efficiency. Thus, this advanced O substituted g-C3N4 casts significantly enhanced photocatalytic hydrogen evolution activity, achieving similar to 9-fold enhancement compared to bulk g-C3N4 under visible light irradiation with a high apparent quantum efficiency (AQE) of 13.2% at 420 nm, exceeding most of the reported doped g-C3N4. Besides, this synthetic route is also applicable to preparation of O substituted g-C3N4 with other precursors, such as urea and thiourea, highlighting the universality of the current strategy. The study offers a new perspective into exploitation of high-performance catalysts for solar energy conversion with a facile and general decoration strategy.