Abstract: In order to explore the process of nitrogen displacing gas in abandoned roadway of coal seam, and understand the state of diffusion motion of nitrogen in the process of displacing gas in the abandoned roadway. The roadway numerical model was established, the software of Fluent was used to simulate and analyze the state of nitrogen during the displacement process. The analysis results show that the displacemnt process of nitrogen is linear and slow diffusion in the initial phase of nitrogen injection. But, with time going by and the increasing of the amount of nitrogen injection, the injected nitrogen and the gas in the original abandoned roadway present convection and diffusion state; it forms the complete displacement of nitrogen and the mixed displacement segment, and is gradually moving forward. Combining with Engineering case of continuous injection of nitrogen to displace gas in abandoned roadway of Detong Coal Mine, the effectiveness and stability of nitrogen displacing gas were investigated.