This paper pointed out that the basic reason of air preheater’s corrosion and blocking in the selective catalytic reduction (SCR) flue gas denitrification process was ammonia escape. The influence of flow field distribution, automatic regulation for the ammonia spraying and flue gas analysis instruments on the ammonia escape were investigated, in order to suggest the method of reducing ammonia escape. The result shows that the grid nozzle for ammonia spraying should be improved.