In the combustion process of lean coal boilers，in order to ensure the ignition，stable combustion and burnout of lean coal，the conditions such as temperature and oxygen content in the combustion process must be fully considered and guaranteed，which is obviously contradictory to the low oxygen and low temperature conditions required for NO_x emission control to a certain extent. Therefore，for the low nitrogen combustion technology of lean coal boiler，how to ensure the low nitrogen combustion without affecting the combustion performance of boiler has always been the difficulty of this technology. Based on the preliminary scheme of low nitrogen combustion technology of boiler，a feasible deep optimization transformation scheme of low nitrogen combustion technology was put forward through system analysis and design. In this paper，for a 300 MW lean coal boiler，the effects of air classification low-nitrogen combustion mode and the area of the bottom secondary air nozzle on NO_x emissions were analyzed through numerical simulation. Based on the preliminary plan of adding three layers of new SOFA wind to the boiler for low-nitrogen combustion reform，through further calculation and analysis，a low NO_x combustion depth optimization for increasing the area of the bottom secondary air nozzle by 1.5 times was proposed. The simulation results show that the NOx emission is reduced from 473.4 mg/m³ to 265.3 mg/m³. The NO_x emission can be reduced by more than 40% after the transformation，and the temperature field and oxygen field of the boiler can be evenly distributed. After the transformation，the actual operation results on site show that the NO_x emission is reduced from 481.6 mg/m³ before optimization to 269.1 mg/m³ after optimization，and the emission of nitrogen oxides is reduced by 44.1%，which has realized the safe，high efficient and low pollution operation of lean coal boilers.