Display omitted •Dicyandiamide is transformed into melamine-cyanaurate complex (MCA) under hydrothermal process.•MCA leads to the formation of mesopores g-C3N4 nanosheets under optimized conditions.•The g-C3N4 nanostructure shows a high hydrogen evolution rate of 136.9 μmol/h.•Its apparent quantum efficiency (AQE) is as high as 10.6% at 420 ± 15 nm. Annealing a supramolecular precursor with a similar structure to the local arrangement over the g-C3N4 building blocks at molecular level is a simple and effective route for preparing g-C3N4 nanostructures. Herein, a facile process for synthesizing the supramolecular precursor is presented, from which the uniform 3D holey g-C3N4 nanosheets with considerably high photocatalytic performance are obtained. The precursor synthesized from hydrothermal treatment of dicyandiamide presents a similar structure to of that the melamine-cyanaurate (MCA) complex prepared by the assembly of melamine and cyanuric acid, while the differences in thermal decomposition and morphology exist. Consequently, the g-C3N4 samples obtained by annealing the two precursors obtained via the different synthesis routes exhibit district microstructures, optical properties, charge recombination and photoelectrochemical behavior, and photocatalytic activity. It is found that the g-C3N4 nanostructure prepared by annealing the hydrothermal product of dicyandiamide shows the hydrogen evolution rate of 136.9 μmol/h, which is 2.5 folds that of the one obtained from the conventional MCA complex and 15-folds of bulk g-C3N4, and its apparent quantum efficiency (AQE) reaches 10.6% at 420 ± 15 nm.