Two different types of coal-fired units are selected to compare the NO_xconcentration,the ammonia escape rate and the NO_xconcentration of the SCR denitrification device outlet and the chimney inlet measured by CEMS before and after the ultra-low transformation.At the same time,on the ultra-low emissions after the transformation of the air preheater dust and catalyst samples are analyzed.The results show that after the ultra-low emission of the unit,there are some problems that the bad uniformity of the NO_xconcentration distribution and the increase of the escaping ammonia concentration,resulting in serious ammonium hydrogen sulfate blockage in air preheat heat.Catalyst active component of velocity is relatively serious,sulfate and alkaline earth metal elements content increased significantly.In order to solve the above problems,it is proposed to adopt multipoint monitoring,spray ammonia optimization,catalyst regularly tested,air-conditioner cold-end replacement of enamel-enriched elements and other measures.