Considering the damage effect of porous media under triaxial compression, the calculation formula of effective stress is modified. It is verified that the formula can very well describe complete stress-strain process of gas-filled coal under triaxial compression. The permeability experiments under triaxial compression show that the relationship between the effective volumetric stress and permeability of gas-filled coal samples is negative exponent before the peak stress. It is still exponentially after the peak point, but the coefficient changes. The permeability of the gas-filled coal samples changes in reverse direction with the effective volumetric stress, that is, the effective volumetric stress increases when the permeability decreases, and vice versa.