Abstract: The technical retrofit adopting low NO_x combustion technology was carried out for a 1 000-MW ultra supercritical boiler to reduce the NO_x emissions. After the retrofit, the NO_x emission mass concentration was down by 42%, while the CO emission mass concentration and the carbon content of fly ash were higher. Therefore, based on the analysis of the variation of the combustion conditions before and after the retrofit, the combustion adjustment test was conducted, in which the boiler combustion and NO_x emission characteristics under different working conditions were studied by changing the oxygen volumetric fraction at the economizer exit, the secondary air distribution mode, the OFA flow rate and the SOFA flow rate. The test results show that the CO emission mass concentration has been dropped to less than 100 mg/m³ and the carbon content of fly ash has been down below 1% under high boiler load after the combustion adjustment.