Core-shell structure ZnO/g-C₃N₄ nano-photocatalysts with ultrathin amorphous g-C₃N₄ layer were synthesized via facile polymer network gel method applying zinc oxide and dicyandiamide as the raw materials. When the molar ratio of zinc oxide to dicyandiamide was 1∶1, the heterojunction interface of the composite with ~3 nm thickness of shell layers was distinct, and it had the best photocatalytic activity for the degradation of organic dye pollutants under simulated sunlight and visible light irradiation. Ultraviolet-Visible diffuse reflection spectra (UV-vis) results show that the composite materials have increased absorption in the visible light region, as well as the absorption bands are red shifted. Photoluminescence (PL), Stable Surface Photovoltage (SPV) and Stable Surface Photocurrent (SPC) reveal that the loading of g-C₃N₄ significantly improve the separation of photo-generated carriers. The photocatalytic degradation of Methylene Blue (MB), Methyl Orange (MO) and Rhodamine B (RhB) was carried out under simulated sunlight and visible light. The results exhibit that the photocatalytic activity of ZnO/g-C₃N₄ composite is effectively enhanced, and the catalyst has excellent stability and reusability. In addition, sacrificial agent experiments proclaim that superoxide radicals are the main active species for composite to degrade pollutant dyes, while holes play an important role in the degradation process. During the photocatalytic process, the core-shell structure can effectively promote the charge transfer between ZnO and g-C₃N₄. This study provides a simple strategy for the preparation of high-efficiency nano-photocatalysts based on graphitic carbon nitride/metal oxide heterostructure.