The solar-to-hydrogen conversion via semiconductor-based photocatalytic water splitting has triggered the search for cost-effective, high-performance photocatalytic materials. Graphitic carbon nitride (g-C3N4) has proven to be a promising metal-free photocatalyst for the hydrogen evolution reaction (HER), however, bulk g-C3N4 suffers from limited HER efficiency owing to its disadvantages including fast electron-hole recombination, low conductivity and irregular microstructure. Herein, we report a supramolecular chemistry-based approach to construct g-C3N4 nano-architectures by calcining the pre-organized complexes originating from the copolymerization of two symmetrical precursors, melamine and trithiocyanuric acid. The resultant g-C3N4 micro-/nanostructures not only effectively induce enhanced visible light-harvesting property but also accelerate the electron-hole separation and charge carrier transfer, leading to highly improved HER performance. X-ray photoelectron spectroscopy (XPS) analysis confirms the chemical environment of different elements, while in-situ electron paramagnetic resonance (EPR) characterizations reveal the types of oxygen-containing radicals and charge carrier dynamics. Compared to pristine g-C3N4, the optimal large-aspect-ratio g-C3N4 sheets derived from ethanol show the fastest HER rate of 1144 mu mol.h(-1).g(-1). The improvement of photocatalytic activity can be ascribed to synergistic effects of extended visible light absorption, boosted charge transfer and more active catalytic sites for HER in modified g-C3N4 materials.