Abstract: When a 300 MW bituminous coal fired boiler in a certain unit was operating at a deep peak load of 30%, the NO_x concentration at the furnace outlet exceeded 650 mg/m³, resulting in an increase in denitrification ammonia injection. Due to the low load flue gas flow rate and uneven flue gas flow field of the unit, the denitrification ammonia escape rate increased relatively more, causing the air preheater to be blocked by ammonium bisulfate, seriously affecting the safe, environmental and economic operation of the unit. In response to this issue, an analysis of the reasons for the high NO_x concentration at the furnace outlet was conducted, and experimental optimization work was proposed to reduce the operating oxygen content and level the first wind heat state. The NO_x concentration at the furnace outlet was significantly reduced, accumulating experience for reducing the NO_x concentration at the furnace outlet under deep peak shaving conditions of boilers.