Abstract: In this paper, we applied a test system of the gassy coal mechanical characteristics established by Dr. Hu and carried on the complete stress-strain experiments and adsorption expansion experiment of gassy coal. The results of experiment show that on the condition of the same confining pressure, mechanical properties of gassy coal are connected to gas pressure. With the higher of gas pressure, the elastic modulus and compressive strength of coal becomes lower, and the ability to resist deformation becomes weaker. With the stronger of gas absorption in coal, the effect of coal mechanical properties gets bigger and the ability to resist deformation becomes weaker; under the condition of the same axial load and confining pressure, with the higher of gas pressure, the rate of coal swelling becomes faster and the demotion quantity when final destruction happens to coal becomes smaller; the existence of free and absorbed gas accelerates the splitting failure of crack and makes coal prone to turn out shear and slip destruction.