On the basis of the polymer reference interaction site model (PRISM), a theoretical approach was proposed to describe gas solubilities in atactic polystyrene (aPS) melt. In the PRISM, our recently developed multisite semi flexible model was applied to derive intramolecular correlation functions. The intermolecular correlation functions for gas gas and gas aPS were obtained and further applied to calculate the excess chemical potential of gas aPS system. The solubility coefficients of a wide range of gases in aPS were predicted at low pressure. These predictions are in good agreement with the experimental data. Particularly, the sorption curves of CO2 in aPS under high pressures were calculated, and the reasonable results were obtained. These results demonstrate that the theoretical model is reliable to describe the sorption properties of small molecules in aPS chain. On the basis of the CO2 sorption isotherms, the swelling of aPS under high pressure were also discussed.