Abstract: In order to study the desorption law of gas in coal particle, the dynamic gas desorption experiment with the changing external pressure in constant temperature was designed. The experiment of gas desorption in coal sample in the environment of changing pressure was performed in this paper. It was obtained that the change curves of cumulative gas desorption amount of different size coal over time under various initial pressure, and the mathmatical model of cumulative gas desorption amount over time was developed. Results show that the reciprocal of coal particle gas accumulation desorption quantity and time function (f (t)=1/tⁿ,0 <n≤1) presents a linear relationship; the greater of initial gas pressure is, the greater gas accumulative desorption quantities; the largest gas desorption quantity of lump coal and its size has no obvious changes in the relationship.