During the actual operation of the power plant, it was found that the addition of water vapor can reduce the amount of denitrifying agent used in the SNCR denitration reaction, which has a certain promotion effect on denitrification and also promotes the combustion process. Therefore, studying the influence and mechanism of water vapor on various denitration technologies is of great significance for achieving low-cost and high-efficiency denitration. This paper analyzed and summarized relevant literature reports focusing on the promotion effect and mechanism of water vapor in the denitration reaction. The study found that water vapor indeed can promote the denitration of SNCR, reburning and advanced reburning technologies, but there is no consistent conclusion on the optimal amount of water vapor. The denitration efficiency improvement of adding water vapor in biomass reburning and its advanced reburning were a little smaller than that in pulverized coal reburning, and advanced reburning. The addition of water vapor can effectively promote the generation of OH groups in the SNCR denitration reaction, which can react with the NH3 reducing agent to generate NH₂ groups, thereby improving its denitration efficiency. The promotion effect of water vapor on reburning is mainly through the formation of OH groups, which increases the volatiles, especially the precipitation of HCN, NH₃, etc., thereby promoting NO reduction. Adding water vapor can increase the denitrification efficiency of advanced reburning technology to more than 80%, it is considered that the denitration mechanism of advanced reburning of biomass by water vapor is similar to the denitration mechanism of reburning, while, the mechanism of advanced reburning of coal powder by water vapor has not been reported. In summary, the influence mechanism of water vapor on advanced reburning should include the both SNCR and the fuel reburning and the interaction between them. However, the current research has not paid attention to the influence on the solid fuel reburning, especially the effects on reduction of NO by coke. Therefore, the influence mechanism and operating conditions of water vapor on advanced reburning and the best reaction conditions should be further studied to provide a strong theoretical support for the application of water vapor in practical.