Abstract: Ozone deep oxidation combined with wet absorption technology has been successfully applied to many sets of ultra-low emission treatment projects for large-scale sintering flue gas, and running stably for many years. However, there is no consensus on whether the oxidation products can be efficiently transmitted from the gas phase to the liquid phase, and whether the nitrogen balance can be realized when the flue gas passing through the absorption system. Therefore, one ozone deep oxidation combined with wet absorption project for 300 m² sintering flue gas in an iron and steel company was taken as an example, and this study established a nitrogen balance research method with clear nitrogen balance boundary, specific nitrogen input, output and accumulation items, and less dynamic parameters. Based on this method, the nitrogen balance was studied. The results showed that the nitrogen conversion rate of the system was 96.4%, which showed that NO_x in flue gas could be efficiently absorbed after ozone deep oxidation. Gaseous NO_x entered and discharged from the desulfurization and denitration system with flue gas, NO₃^- entered the desulfurization and denitration system accompanying flushing water or discharged accompanying gypsum, NO₃^- accumulated in the absorption tower and buffer tank were the most critical nitrogen input, output and accumulation items, which were also the main items to be considered in the actual project.