With the development of industry and agriculture, the problems of environmental pollution and energy shortage have become increasingly severe. Semiconductor photocatalysis technology is one of the effective ways to solve environmental pollution and energy crisis. The principle of photocatalysis is based on the oxidation-reduction ability of photocatalysts under light conditions, which can achieve the purposes of purification of pollutants, material synthesis and transformation. Graphite phase carbon nitride (g-C3N4), as a new high-efficiency catalyst, has good stability and shows great engineering application potential in photocatalytic technology. However, the unmodified g-C3N4 has a limited visible light response range, and the photo-excited charge carrier recombination rate is high, resulting in low photocatalytic activity. Nitrogen defects are introduced into the g-C3N4 framework. These defects can manipulate the electronic structure, and the interstitial state produced can be used as a band-tail state, which can overlap with the valence band or the conduction band. The mid-gap state of semiconductors can extend the light response and act as an active center for electron-hole excitation. Introducing defects into g-C3N4 can improve the photocatalytic activity of g-C3N4. This paper systematically reviews the physical, chemical and electrochemical properties of g-C3N4 on the basis of experimental and theoretical research progress. The synthesis methods of defect g-C3N4 are summarized, including adjustment before polymerization, adjustment during polymerization, and adjustment after polymerization. The adjustment before polymerization is to introduce defects by changing the precursor, such as adding hydroxide, sodium borohydride and other substances to the precursor. The adjustment during polymerization is to provide a reducing atmosphere during polymerization can prepare g-C3N4 with different nitrogen-vacancy densities, such as hydrogen, ammonia and so on. The adjustment after polymerization is to modify the synthesized defect-free g-C3N4, such as recalcining or acid treatment to achieve the purpose of synthesizing nitrogen vacancies. The effect of defect sites on g-C3N4 is also discussed. The intermediate state induced by nitrogen defects can be transformed into active centers excited by electron holes, and the optical response of defect g-C3N4 is broadened due to the narrowing of the band gap. In the range, nitrogen defects can trap photo-generated carriers and prevent their recombination, thereby increasing the overall quantum efficiency. However, excessive introduction of nitrogen defects will produce deeper interstitial states. These deeper interstitial states can not only capture photo-generated electrons, but also photo-generated h+, which then become the recombination sites of photo-generated carriers. In addition, we separately summarized the application of the defect g-C3N4 in water treatment, such as the degradation of antibiotics and organic pesticides and the reduction of the toxicity of heavy metals, as well as water decomposition, carbon dioxide conversion and photocatalytic denitrification. Defect g-C3N4 has achieved good results in these applications. Although considerable progress has been made in the research of g-C3N4 in recent years, there are still many challenges in preparing g-C3N4 with high-efficiency catalytic performance. Finally, in view of the challenges faced by the application of defective g-C3N4, key discussions and future prospects are proposed from the aspects of mechanism exploration and material development.