Developing energy-saving absorbents for carbon dioxide (CO2) is essential for improving carbon capture and storage (CCS) technologies. Recently, we have designed phase separation solvents, which can significantly reduce the regeneration energy for CO2 capture and separation down to 1.6 GJ/ton-CO2 [Machida, H.; Int. J. Greenhouse Gas Control2018, 75, 1-7]. For further developing better solvents, this paper studied a theoretical approach with conductor-like screening model for real solvents (COSMO-RS) to screen the amine/ether/water systems to identify phase separation solvents upon CO2 absorption. In this work, liquid-liquid equilibria of 21 amine/ether/water systems were determined both before and after CO2 absorption. Experimentally, it has already been demonstrated that the phase behavior of these systems is classified into three categories: phase separation type, miscible type, and immiscible type. This study demonstrates that the octanol/water partition coefficient is empirically able to search for a combination of amine and ether compounds, in which the system exhibits the phase separation behavior by absorbing CO2. The COSMO-RS calculations successfully reproduced experimental phase behavior with a rate of agreement of more than 80% by accounting the following two factors: (i) ion pairing for the description of the association/dissociation state for ionic species and (ii) relevant low-lying conformations of ether and amine, which are rationalized by experiments such as conductivity and excess enthalpy measurements. Moreover, we also validated the ability of the COSMO-RS calculation to qualitatively describe the compositions of 2-(ethylamino)ethanol, diethylene glycol diethyl ether, and water in CO2-rich and CO2-lean phases at varying CO2 loading conditions.