Abstract: The 3^th generation oxy-rich combustion technology, that is oxy-steam combustion with O₂ and H₂O as working medium, has recently attracted much attention due to its great potential and stability in the field of CO₂ capture. Given that the effect of H₂O on coal ignition and combustion is complex and non-linear, it is still unclear about the effect of H₂O on coal-nitrogen evolution, particularly in high H₂O concentration. In this case, the traditional low-nitrogen combustion technology would not be suitable for O₂/H₂O combustion, increasing the risk of excessive nitrogen oxide (NO_x) emissions. In this paper, the migration and transition of organic nitrogen during coal devolatilization in O₂/H₂O combustion has been studied by using the ReaxFF molecular dynamic (MD) simulation and combining atomic labeling method, emphasizing on the migration route of nitrogen-containing functional groups and the formation of nitrogen-containing precursor. This will help to develop the technology of low-nitrogen combustion for the oxy-steam combustion. Results show that in the initial stage of coal devolatilization, the presence of high H₂O concentration greatly promotes the formation of hydroxyl group (e.g. OH radical and C—OH group). It is beneficial to break aromatic ring and to release nitrogen-containing precursors, but it is not obvious in the initial stage of coal devolatilization. The formation of a large number of hydroxyl groups promotes the conversion of quaternary nitrogen (N-Q) in the aromatic ring to pyridine nitrogen (N-6) at the carbon boundary. In the meanwhile, the migration of N-6 and pyrrole nitrogen (N-5) to oxidized/amorphous nitrogen (N-X) is intensified. In the initial stage of coal devolatilization, the solid products obtained at high humidity have higher contents of N-6 and N-X, a lower stability of nitrogen-containing functional groups in coal, as well as a higher activity of reactivity sites. As time goes on, the presence of H₂O and its resulting groups change the source of nitrogen-containing precursors. The conversion rates of N-6 to HCN and N-5 to NH₃ are increased, but have little influence on the yields of HCN and NH₃.