Abstract: The determination of coal strength is usually done under non-adsorption condition in the laboratory. To reveal the variation of coal strength with gas pressure under adsorption condition, based on gas control theory and field practice, an automatic test device for the strength coefficient of coal in closed inflatable environment was developed, the influence of gas pressure on coal strength under adsorption condition is studied. Experiments show that the strength coefficient of coal is obviously affected by the gas pressure of coal sample, the greater the gas pressure is, the smaller the strength of the coal is, and the less able to resist external force breaking.