In this work, a priori COSMO-SAC model was combined with the original UNIFAC model, and thus the new COSMO-SAC-UNIFAC thermodynamic model with a strong predictive power for fluid phase equilibrium (VLE, LLE) was proposed. By this means the group binary parameter matrix of original UNIFAC model was extended by introducing the new 648 vacant parameter pairs for 51 main functional groups for the conventional substances in this work. Moreover, the combined thermodynamic model was first applied to process simulation on gas drying with ionic liquids (ILs). To verify the reliability of the COSMO-SAC-UNIFAC model, the predictive values were directly compared with experimental data coming from this work, our previous work, and literature. The moderately accurate predictions of the COSMO-SAC-UNIFAC model demonstrate the high potential applicability of this new model, especially for numerous systems with missing parameters including ILs, which are normally encountered in the original UNIFAC model.