In order to meet the ultralow emission standard of NOx, the combustion system of a 600 MWe tangentially-fired lignite boiler was retrofitted using air-staged combustion technology. The retrofit scheme is described in detail. Combustion and NOx emission characteristics of the original and retrofitted boiler were numerically studied and compared. Optimization simulations for retrofitted boiler were carried out under different over-fired air ratios. The moisture content in lignite was specially considered in simulations by simplified coal compositions method. The results show that the ignition of lignite particles is delayed due to the high moisture content. After retrofit, the temperature level in furnace rises owing to the reduction of total excess air coefficient in furnace, and the high temperature region moves upward due to the introduction of separated over-fire air. The NOx emission is reduced by approximately 40% through retrofitting, which proves that the low-NOx retrofit for the lignite-fired boiler is successful. With over-fired air ratio increasing from 15% to 25%, the NOx emission for retrofitted design decreases from 394 mg/Nm3 to 317 mg/Nm3 (at 6% O2). OFA ratio of 20% is the most economical case, and OFA ratio of 25% is the most environmentally friendly case. Taking NOx emission and boiler thermal efficiency into account, the practical operating condition with OFA ratio of 20% is optimal for retrofitted boiler. The results can be beneficial for the retrofit and practical operation of similar lignite-fired boilers. •A tangentially-fired lignite boiler was retrofitted using air-staged combustion technology•Combustion and NOX emission in the 600MWe tangentially-fired lignite boiler were numerically studied.•The moisture content in lignite was specially considered by a simplified coal compositions method.•After boiler retrofit better NOX emission performance was achieved.•The optimal OFA ratio in retrofitted boiler was obtained by optimization simulation.