Equations of state (EOS) can be used to construct solubility model of gases in aqueous solutions. A thermodynamic model is introduced to predict the solubility of methane(CH4), hydrogen sulfide(H2S), and carbon dioxide(CO2) in pure water and in aqueous salt solution over a large range of temperature, pressure and salt concentrations (273-523K, 1-2000 bar, 0-6m for CH4; 273-500K, 0-200bar, 0-6m for H2S; 273-533K, 0-2000bar, 0-4.5m for CO2), with accuracy equivalent to that of reliable experiments. With the specific interaction approach, this model is not only able to predict gas solubility in NaCl solutions, but also other more complex systems, such as seawater and geothermal brines, without fitting experimental data from these systems. The model find wide applications in the study of solubility of gases in nature aqueous solutions with different temperatures, pressures and salt conditions, and this model can also analyze the PVTX conditions of fluid inclusions, calculate the immiscibility of mineralization fluids, the minimum conditions of CH4 hydrate’s formation, and even CO2 sequestration. Online calculation of the solubility of these gases is made available on: www.geochem– model.org/models.htm.