Abstract: In order to ensure the experiment of coal seam gas degradation by methane-oxidizing bacteria smoothly, we developed the high pressure gas mixing device, researched the laws of coal seam gas degradation by methane-oxidizing bacteria and the combined effects of coal adsorption gas capacity under different pressures, and aerobic or anaerobic conditions. Through detecting, contrasting and analysis the changes of the amount of CH₄ and CO₂, we found that the higher pressure is good for coal seam gas degradation by methane-oxidizing bacteria between 1 MPa and 5 MPa pressure range; under equal pressure condition, the reduction of methane in aerobic environment is more than anaerobic.