A facile “one-pot” method was adopted to engineer delocalized π-electron and Donor-Acceptor (D-A) system in polymeric carbon nitride (PCN). The resulting UCN-xTAB materials demonstrate an expanded light absorption range due to the expanded delocalization with the graft of 1,3,5-benzenetriamine (TAB) into the CN network. The constructed D-A system further promote the exciton dissociation and charge migration. Consequently, the optimized samples UCN-5TAB displays a remarkable apparent quantum efficiency (AQE) (20.2 %) for H production at 450 nm, surpassing the performance of previously reported PCN-based photocatalysts. The proposed structure for UCN-xTAB was confirmed by Density-functional-theory (DFT) calculations. In addition, the regulation of delocalization can be probed by tuning the incorporated monomers (melamine, 2,4,6-triaminopyrimidine), demonstrating the significance of delocalization for enhanced photocatalytic activity. This work provides a deep understanding on the role delocalization and D-A structure of PCN for efficient photocatalytic activity with improved solar harvesting in long wavelength region.