C₃N₅ as a two-dimensional (2D) layered polymer material has great prospects in the field of energy storage due to the excellent light absorption, low electron transfer resistance, and environmental friendliness, etc. However, several drawbacks such as the high charge carrier recombination rate, weak reduction ability and low density of surface reactive sites give rise to a poor photoactivity, restricting the large-scale application. Recent researches focus on the synthesis and the detailed molecular structure of C₃N₅, as well as the various modification strategies to promote the photocatalytic activity. It is thus necessary to provide a general guidance for designing high-efficient C₃N₅ catalyst systems based on the existing results.C₃N₅ is commonly prepared by thermal polymerization method in the presence of organic materials precursors that contains a large amount of nitrogen element like melem hydrazine, 3-Amino-1,2,4-Triazole, 5-Amino-1h-Tetrazol, etc.C₃N₅ has three structures like triazine-triazole structure, azo structure and terminal triazole structure. The unique structures and bonding types of C₃N₅ endow it with a promising possibilities for photocatalytic applications.Various modification strategies including morphology control, nitrogen vacancy creation, element doping and heterojunction construction in promoting the photocatalytic activity of C₃N₅ are discussed. Morphology control is beneficial to improving the specific surface area and enhancing the density of surface reactive site of C₃N₅. Nitrogen vacancy and element doping favors optimizing the band structure and improving the utilization of solar energy. Heterojunction construction like the Schottky junction, type-Ⅱ, Z-scheme and S-scheme C₃N₅-based heterojunctions enable the efficient spatial separation of charge carriers and maintain the intense redox capabilities.Summary and prospectsC₃N₅ as a two-dimensional (2D) layered polymer material has advantages such as the unique structures, larger amount of nitrogen active sites, narrower band structure and excellent chemical stability. It is more favorable to the development prospects of C₃N₅ in various photocatalytic fields. This review summarizes the synthesis and molecular structure of C₃N₅, and the modification strategies developed to improve the photocatalytic performance of C₃N₅ nanomaterials in recent years, including morphology control, nitrogen vacancy creation, element doping and heterojunction construction. However, compared with the abundance of other semiconductor catalyst systems, a research on C₃N₅ still needs to be further explored in terms of rational preparation, construction of unique composite systems, elucidation of the intrinsic mechanism, and the application areas, etc.In rational preparation, the eco-friendly and cost-effective large-scale preparation of C₃N₅ nanomaterials is still a challenge. The relationship between the reaction parameters in the thermal polymerization process and the resulting morphology structure is unclear, which hinders an ability to optimise and control the process further. Also, the synthesis of C₃N₅ inevitably results in the formation of toxic by-products, necessitating the development of supplementary follow-up treatment technology. The optimization of the rational synthesis of C₃N₅ for environmentally and large-scale preparation is more in line with sustainable development strategies.In the construction of unique composite systems, the precise regulation is a pivotal concern, both in terms of industrial applications and fundamental scientific research. Until now, it becomes a challenge to elucidate the precise conformational relationship between vacancy/elemental sites density and photocatalytic performance, representing a significant obstacle to further performance optimization. The advancement of the precise regulation is instrumental in enhancing the catalytic performance of C₃N₅-based materials.In the elucidation of the intrinsic mechanism, it is essential to investigate the atomic and electronic structures at the material/interface level in order to clarify the performance of the reaction process. This provides a theoretical foundation for the subsequent design and development of efficient photocatalysts. It is thus necessary to employ advanced characterizations like environmental transmission electron microscopy, aberration scanning transmission electron microscopy, synchrotron radiation and in-situ spectroscopy. The structural configurations for catalysts and the formation of intermediates during photoreaction can be monitored. The fine update is more beneficial to optimizing the properties of the catalyst material.In the context of application areas,, the optimization of selectivity in catalytic reactions represents a crucial challenge for designing high-efficient photocatalytic systems. In the context of energy catalysis, such as methane conversion and CO₂/N₂ reduction, a detailed understanding of the intrinsic reaction mechanism and kinetic processes elucidated by theoretical simulation and in-situ monitoring is beneficial for the design and development of C₃N₅ catalysts with a high selectivity and a purpose of improving the yield of the target product. In the context of environmental remediation, the utilization of wastewater and seawater instead of purified water for hydrogen production from water is another way to achieve a sustainable development strategy. The design and development of C₃N₅-based materials with a high selectivity can facilitate their broader range of applications.