Efficient charge separation and broaden light absorption are of crucial importance for solar-driven hydrogen evolution reaction (HER), and graphitic carbon nitride (g-C3N4) is a very promising photocatalyst for this reaction. Here we report a facile precursor pre-treatment method, by forming hydrogen bond-induced supramolecular aggregates, to fabricate g-C3N4 with simultaneous novel porous network and controllable O-doping. Experimental and DFT computation identified that O doping preferentially occurs on two-coordinated N position, and the porous network and O-doping synergetically promote the light harvesting and charge separation. As a result, this material shows 6.1 and 3.1 times higher HER activity (with apparent quantum efficiency of 7.8% at 420 nm) than bulk and even 3D porous g-C3N4. This work highlights that simply pre-treating the precursor can not only control the architecture but also introduce helpful foreign atoms or monomer in the matrix, which provides a useful strategy to design and fabricate highly efficient g-C3N4 photocatalyst.