Development of low-cost and efficient photocatalytic materials with visible-light response is of urgent need for solving energy and environmental problems.Here,a metal-free two-dimensional (2D) π-conjugated hybrid g-C3N4 photocatalyst with tunable band structure was prepared by a novel one-pot bottom-up method based on a supersaturated precipitation process of urea and triethanolamine (TEOA)solution.The microstructure of the hybrid g-C3N4 is revealed to be a compound of periodic tri-s-triazine units grafted with N-doped graphene (GR) fragments.From experimental evidence and theoretical calcu-lations,the two different π-conjugated fragments in the hybrid g-C3N4 material are proved to construct a 2D in-plane junction structure,thereby expanding the light absorption range and accelerating the inter-face charge transfer.The π-conjugated electron coupling in the 2D photocatalyst eliminates the grain boundary effect,and the coupled highest occupied molecular orbital (HOMO) effectively promotes the separation of photo-induced charge carriers.Compared with the g-C3N4 prepared by the conventional method,the visible-light H2 production activity of the optimized sample is enhanced by 253 ％.This work provides a new strategy of constructing metal-free g-C3N4 hybrids for efficient photocatalytic water splitting.