As an artificial photosynthesis, semiconductor photocatalysis technology can alleviate the current global environmental pollution and energy shortage crisis, and has become a hot research direction in recent years. Graphitic carbon nitride (g-C3N4) is a two-dimensional layered semiconductor material, which has a simple preparation method and good visible light response ability, and is currently the research focus of photocatalytic materials. The photocatalytic performance of g-C3N4 prepared by direct polymerization of nitrogen-containing precursor system is poor, so it is necessary to regulate the performance of g-C3N4. In this paper, several main preparation methods of g-C3N4 are briefly introduced. Then, it focuses on the modification methods of g-C3N4, including improving crystallinity, constructing heterojunction, regulating morphology and defect engineering, and summarizes the application of g-C3N4 in the field of photocatalysis, including degradation of organic pollutants, decomposition of water to produce hydrogen, production of H2O2 and reduction of CO2. Finally, the future development of g-C3N4 is prospected, and it is suggested that while continuously improving the photocatalytic performance of g-C3N4, more consideration should be given to the design problems faced by actual industrial production. Further research on the preparation of highly crystalline g-C3N4 by molten salt method, the construction of g-C3N4-based heterostructure system, the clear separation mechanism of photogenic carrier in g-C3N4, the mechanism analysis of adsorption and REDOX reaction of reactants on g-C3N4 surface, and the research of recoverable g-C3N4-based photocatalyst are taken as the key development directions. The study on the modification of g-C3N4 has deepened our understanding of the mechanism of photocatalysis, and provided an efficient and stable photocatalyst candidate material for practical applications such as environmental purification and energy conversion, showing a broad application prospect.