In view of the situation that the permeability of the most high gas coal seam is poor and the effect of coalbed methane extraction is not ideal by strengthening extraction in low permeability coal seam, which combines the strong diffusion, dissolution-increasing-permeability on pore medium and other unique advantages of super-critical carbon dioxide. According to the evolution features of coal pore fissure after the action of supercritical carbon dioxide, the micro porosity and permeability evolution equation was gained, the damage variable was ascertained according to the change of the porosity. Thermo-fluid-solid coupling model of coal was built after the action of supercritical carbon dioxide, which considered the effects of volume stress, temperature, pore pressure and the dissolution-increasing-permeability action of supercritical carbon dioxide, field variable subroutine supplied by ABAQUS was used, the PYTHON script and the two development function of subprogram were combined, the increasing-permeability law of numerical simulation is realized in the low permeability coal seam injected by supercritical carbon dioxide. The results showed that after the injection of supercritical carbon dioxide, the volume stress, temperature and pore pressure around the injection-gas hole changed obviously, with the increase of the distance from the injection hole, the influence degree gradually weakened and tended to be stable. New pore fissures sprouted constantly and interconnected with the original hole fissures in the coal around the injection hole, the crevice at all levels evolved constantly to the depth of coal, the micro-porosity was increased by two orders of magnitude than that before injecting the gas. The fracturing and increasing permeability action of supercritical CO2, the coal was damaged with different degrees, the closer the injection-gas hole is, the greater the degree of damage is and the faster the damage increases, the longer the gas injection, the greater the increase in damage. Effective development of micro pore and fissure of coal, more transport channels were provided for the diffusion and seepage of coal bed methane, the permeability coefficient of coal was increased by three orders of magnitude than before gas injection.
