A broad bandgap and detrimental recombination of photoexcited h(+)-e(-) pairs are fatal deficiencies for using the g-C3N4 visible light water splitting. Herein, a sea-urchin-structure g-C3N4 (CNSC) with similar to 2.0 eV bandgap was prepared using a hydrothermal strategy. It is important to note that CNSC can efficiently suppress h(+)-e(-) pair recombination and has a narrow bandgap which can utilize more visible light. Conventional g-C3N4 has a similar to 2.7 eV bandgap and valence band (VB) potential of +1.83 V vs. NHE but the VB of CNSC is +1.55 V. As a result, CNSC does not create H2O2, which can passivate g-C3N4. Density functional theory (DFT) confirms that C N, C=O and -OH groups in the CNSC shift the d-band centre of Pt closer to Fermi level, leading to better stabilization of adsorbate and higher catalytic performance. Photo-depositing Pt on the CNSC, 3 wt% Pt/CNSC produces H-2 and O-2 evolution rate (HER and OER) of 41.5 and 20.3 mu mol g(-1) h(-1) (apparent quantum efficiency: 0.43% at 420 +/- 10 nm), respectively, 30 times greater than HER of 2 wt% Pt/bulk g-C3N4 in overall water splitting under visible-light (lambda >= 420 nm). This work provides an innovative approach to construct hierarchical nanostructure g-C3N4 with narrow bandgap and paves the pathway for development of water splitting photocatalysts.