The damage growth of coal induced by coal mining and gas exploitation conduces to not only the degradation of mechanical properties, but also the modification of interaction between coal and gas. Considering the swelling deformation of coal during adsorbing gas, the effective stress equation for gas-saturated coal under compression is built up based on damage mechanics and porous media theory. The pore pressure effective stress coefficient and adsorption effective stress related with damage evolution of gas-saturated coal are obtained. Then by using the effective stress equation and linear elastic constitutive model, the deformation and effective stress coefficient of gas-saturated coal are computationally analyzed with the different stress conditions. The computing value fits with the test result quite well, the fact indicates the equation is able to effectively describe the mechanical behavior of gas-saturated coal.