Abstract: The installed capacity of wind power and photovoltaic power is expanding year by year, and the coal-fired units need frequent and deep peak shaving. Under low load conditions, the oxygen content of flue gas will be increased by increasing air and stabilizing combustion and flue air leakage. After the conversion of 6 % dry basis oxygen in the national standard, NO? is prone to exceed the conversion standard, and the contradiction between stable combustion of units and ultra-low emission control is prominent. In order to solve this problem, this paper deduces the calculation model of oxygen conversion, divides the working conditions based on the measured data of the full load of the unit, and systematically combs the causes of the distortion of the converted data. The study found that the flue blowby and low load excessive air supply were the main inducements of the conversion concentration amplification, and the existing fixed reference oxygen assessment mode and deep peak shaving condition had short adaptation boards. The implementation of coordinated management and control by means of flue leakage stoppage, SCR fine ammonia injection, furnace low nitrogen air distribution and other means can give consideration to the safe operation of boilers and environmental protection standards, and provide technical support for the wide load environmental protection operation and maintenance of thermal power units.