Disordered intermolecular interaction carbon nitride precursor prepared by water-assisted grinding of dicyandiamide was used for synthesis of g-C3N4. The final sample possesses much looser structure and provides a broadening optical window for effective light harvesting and charge separation efficiency, which exhibits significantly improved H2 evolution by photocatalytic water splitting. The bottom-up mechanochemistry method opens new vistas towards the potential applications of weak interactions in the photocatalysis field and may also stimulate novel ideas completely different from traditional ones for the design and optimization of photocatalysts.