Abstract: With the increasing consumption of fossil fuels, severe energy shortages and environmental issues are fast approaching. Therefore, the development of green energy resources is urgently appealed. Among them, the sunlight-driven production of hydrogen fuel with suitable photocatalysts is regarded as one of the potential strategies to meet the sustainable energy demand in the future. However, photocatalysis still faces significant uncertainties mainly because of the notorious photogenerated electron-hole (e-h) recombination and low carriers’ mobility. To achieve high photocatalytic performance, it is essential to tailor the spatial charge separation and fast charge transfer via electronic and structural manipulation of photocatalysts. As one of the hot-spot photocatalysts, graphitic phase carbon nitride (g-C₃N₄) has received tremendous attention in the study of solar-to-fuel (STF) conversion and carbon dioxide reduction reactions (CO₂RR), owing to intrinsic merits, such as metal-free components, low-cost resources, good stability, and visible light response. Recently, considerable progress has been achieved to improve the photocatalytic STF efficiency of g-C₃N₄-based materials by developing strategies of structures and electric configurations engineering. In this study, different modification methods for g-C₃N₄ were systematically reviewed from the perspective of defects control to provide a new understanding of its structure-function relationship. Particularly, this study was composed in detail from three aspects to demonstrate the latest research progress of g-C₃N₄ photocatalytic materials. First, different routes toward g-C₃N₄ with different shapes were introduced, including 1D, 2D, and 3D. Second, doping effects and defect control on the separation and transfer of photogenerated electron-hole pairs were carefully reviewed. Finally, heterojunctions based on g-C₃N₄ were summarized, highlighting the Z-scheme heterojunction. In addition, some future directions and challenges for the enhancement of the photocatalytic efficiency upon g-C₃N₄ were pointed out according to our understanding of photocatalytic water splitting.