Abstract: Through the physical simulation experiments of gas injection in coal seam, the mechanism of injecting nitrogen to displace coal seam methane under pressure of 0.6 MPa, 1.0 MPa，1.4 MPa is analyzed. The analysis of experiment results show that at the early stage of gas injection experiment, the injected nitrogen is adsorbed by coal and the methane in coal is replaced. When the injected nitrogen breaks through the box, the displacement effect of nitrogen appears. The macroscopic phenomena of the experiment show the displacement and replacement effect of injected nitrogen. But starting from the methane desorption of micro pore, dilution and diffusion effect could be one of the mechanisms of nitrogen injection to displace coal seam methane. Through the analysis of injecting nitrogen to displace coal seam methane, the mechanism of injecting weak adsorption gas to displace coal seam methane is concluded.