Abstract: At present, researches on soft and hard coal mainly focus on influence of pore structure on its gas adsorption and desorption characteristics, but there are few researches on difference of gas desorption characteristics of soft and hard coal under condition of external moisture. In view of the above problem, the orthogonal experiments under different adsorption equilibrium pressure and external moisture content were carried out by using coal seam high-pressure water injection simulation experimental device, and the differences of gas desorption amount, gas desorption speed and diffusion coefficient of soft and hard coal under different external moisture were analyzed. The results show that with the increase of external moisture content, the gas desorption amount of soft and hard coal decreases, but the gas desorption amount of soft coal is always greater than that of hard coal; the external moisture can inhibit the gas desorption of soft and hard coal, but the inhibition effect of soft coal is stronger than that of hard coal; under different adsorption equilibrium pressure and external moisture content, the gas desorption speed of soft and hard coal is the highest at the initial moment, and it shows a decreasing trend with the extension of time and the increase of the external moisture content; under the same adsorption equilibrium pressure and similar external moisture content, the gas desorption speed of soft coal is always higher than that of hard coal and the diffusion coefficient of soft coal is higher than that of hard coal; the gas diffusion coefficient of soft and hard coal decreases with the increase of external moisture content under the same adsorption equilibrium pressure.