Nitrogen oxides are inevitable hazardous components in coal-fired flue gas. This study designed a series of experiments and combined theoretical calculations to systematically investigate the effect of NOx on the removal of element mercury (Hg0) by nano-amorphous selenium (nano-a-Se). It was found that the impact of NOx on the removal of Hg0 by nano-a-Se primarily involves two mechanisms: competitive adsorption between NOx and Hg0, and the induced reduction effect of NOx on chemisorbed mercury (HgSe). NO inhibits the removal of Hg0 by nano-a-Se, and competitive adsorption is identified as the main influencing factor. Whereas the inhibitory effect of NO2 on the adsorption of Hg0 by nano a-Se can be counteracted due to its oxidizing effect on Hg0. Therefore, although NO2 presents stronger competitiveness than NO in the competitive adsorption with Hg0, it still shows a promoting effect on Hg0 removal, with 50 ppm NO2 restoring 5.7 % of the Hg0 removal efficiency. Additionally, the mechanism of NOx-induced reduction of HgSe was investigated in detail. NO2 is more capable of inducing the reduction of Hg(II) from HgSe to Hg0. This study presents new insights into the underlying influence mechanism, which could provide valuable references for the application of other selenium-based adsorbents.