Abstract: Ammonia escape during flue gas denitrification not only causes waste of resources, but also affects the stable operation of the unit. Due to the easy adsorption and reaction characteristics of ammonia, with the change of flue gas temperature, the form of ammonia escape will change during the subsequent migration process. Indophenol blue spectrophotometry was used to measure the concentration of different forms of ammonia escape at the three positions which are denitrification outlet, air preheater outlet and chimney outlet. TDLAS (Tunable Diode Laser Absorption Spectroscopy) technology combined with in-situ sampling method and in-situ heat and moisture method was used to simultaneously monitor the ammonia escape at denitrification outlet and chimney outlet. The measurement results of indophenol blue spectrophotometry show that when the ammonia escape from the denitrification outlet migrates to the air preheater, part of the ammonia escape is converted into ammonium salt, and the remaining ammonia escape migrates to the chimney total outlet. The measured ammonia escape concentration is greatly reduced compared to that at the denitrification outlet. TDLAS online monitoring results also show that the ammonia escape concentration at the total outlet is much smaller than that at the denitrification outlet. Research has shown that the selection of ammonia escape monitoring points is very important, which determines the accuracy and authenticity of the monitoring results.