Abstract: Thermal decomposition is an effective method to reduce N2O concentration in flue gas. The experiment was conducted with a fix bed reactor. The N2O concentrations before and after reactor under different O2 concentrations were measured and compared. The results show that the increase of oxygen concentration slows the reaction speed of thermal decomposition of N2O. The influence mechanism of oxygen concentration on the thermal decomposition of N2O is simulated with P. Glarborg’s model. The increase of oxygen will reduce the concentrations of H and OH which mainly decide the reaction speed of thermal decomposition of N2O. It is necessary to control the oxygen concentration to increase the de-N2O efficiency by thermal decomposition.