Abstract: SCR denitration process is widely used in China's coal-fired power plants. It is difficult to accurately measure the escape rate of denitration ammonia. The power plant can not accurately grasp the ammonia injection amount of denitration system, which is easy to cause a large number of ammonium bisulfate generation and air preheater blockage. This paper studies the measurement method of denitration ammonia escape rate. Through accurate measurement and calculation of intermediate parameters, the relatively accurate denitration ammonia escape rate is obtained through integrated calculation. It is successfully applied in the diagnosis of air preheater blockage in power plant, and the application points of this measurement technology are put forward.