Graphite carbon nitride (g-C3N4) is a non-metallic semiconductor material with visible light response. g-C3N4 has the characteristics of low cost and easy availability, stable physical and chemical properties, non-toxic and non-polluting, and has good application prospects in the field of environmental purification and energy catalysis. However, the bulk phase g-C3N4 has such shortcomings as small specific surface area, poor visible light absorption ability, and high recombination efficiency of photogenerated electrons and holes, which severely limit its application in practice. In this paper, based on the introduction of the structure, characteristics and preparation methods of g-C3N4, the modification methods of g-C3N4 are mainly summarized, including the research progress of the modification methods such as element doping, morphology control, precious metal deposition and so on. Finally, the reaction mechanism of g-C3N4 in photocatalytic reaction is discussed, and the research of g-C3N4 photocatalyst in the field of purification of water environment is prospected.