Zero-dimensional graphitic carbon nitride nanoparticles (OD g-C3N4 NPs) possess the advantages of non-toxicity, metal-free, and rich surface catalytic active sites. However, the complex preparation process, wide bandgap structure, easily particle aggregation and rapid carriers' recombination still limit their development in photo-catalysis. Herein, these issues are addressed by synthesizing a novel hydroxyl (-OH) modified g-C3N4 NPs with a simple hydrothermal method without using any etching agents. Besides the high hydrophily and small particle size, the -OH decorated OD g-C3N4 NPs possess obvious narrowed bandgap and high reduction potential. To further improve their dispersity and carriers' separation rate, 0D/3D g-C3N4 NPs/ZnS type II heterojunction is fabricated, which simultaneously improves the hydrophily, passivates the surface defects and extends the sunlight absorption range of ZnS. As a result, a high and cyclable photocatalytic activity of 112 mu mol h(-1) (5.6 mmol h(-1) g(-1)) is achieved under visible light irradiation without any co-catalysts, which is 140 times higher than that of pure ZnS and much better than the pure OD g-C3N4 NPs. A systematic study of photocatalytic mechanism is proposed by combining the theoretical calculations and experimental results. This work offers a new sight for the design of OD g-C3N4 NPs based photocatalysts for H-2 production.