Abstract: The gassy soil containing entrapped gas bubbles is a special type of unsaturated soils, and the compressibility of air-water mixture in gassy soils cannot be ignored. Based on the conceptual model for small bubbles, the Bolye’s law of ideal gas and the surface tension effects, equations of the coefficient of compressibility for air-water mixture are derived. When the saturation is higher, the existing equations are not applicable, so a new method for C_gw is proposed, which treats the water and the gas dissolved in the water as a whole. The calculated results under different water saturations are compared with those of other existing methods. The comparitive results show that the proposed method is more reasonable. Finally, the variation characteristics and the impact on the calculated results of the related parameters in the new equation are also analyzed.