Based on the hypothesis of the synthetic coal and gas outburst operating, we study the work done by elastic energy of coal mass and methane expansion energy in the coal and gas outburst process through theory and experiment. By using MTS815 rock mechanics test system, the triaxial compression test of the coal sample is carried out under different confining pressures and the power function relation between coal bodys elastic energy per unit volume and body stress is worked out. It shows there is a linear relation between methane expansion energy and methane pressure obtained by testing methane expansion energy under different moisture contents, different methane pressures and different compactness of coal mass, with the help of self-developed methane expansion energy testing device. The temperature change in the system is about 1 ℃ when methane does work in expansion. The result also shows that it could be regard as an isothermal process during coal and gas outburst. The results from experiment do great help in researching the mechanism of coal and gas outburst and establishing energy criterion.