Abstract: In order to study gas desorption law of No. 9 coal seam in Haizi Coal Mine, we measure gas desorption volume of coal samples under five kinds of gas equilibrium pressure in the laboratory. By analyzing the desorption data, we find that the gas equilibrium pressure has a positive correlation with desorption properties of the coal, and the logarithmic formula can relatively accurately describe the desorption law of the coal seam. Coefficient A of logarithmic formula meets the index relationship with gas equilibrium pressure, and the index formula of this coal seam has already obtained. We can use the relationship of coefficient A and gas pressure to calculate gas pressure of No. 9 coal seam; on-site validation effect is better with higher application value.