A 1000 MW unit USC once-through tower boiler,which had been put into operation in 2012,had a high concentration of 450 mg/m³ NO_x emission at the SCR entrance under the condition of meager coal mixed burning with bituminous coal.Thus it was difficult to realize ultra-low NO_x emission.The paper analyzed the necessity and feasibility on the combustion system retrofit to get the lower concentration of nitrogen oxides.On this basis,the technical scheme,using advanced hybrid air-staged low-NO_x combustion system,was proposed and adopted.The retrofit project was completed at 2016.After the retrofit,the NO_x emission concentration was reduced to 300 mg/m³ at the SCR entrance under the same conditions,boiler efficiency and other indicators were good,also there were no other negative effects.The benefit on economy,safety and environmental protection of the technical retrofit is satisfying.