The aqueous two-phase system (ATPS) containing the polyoxyethylene(20) cetyl ether (C56H114O21, POELE20) and three potassium salts (KOH, K2CO3, and K3PO4) at the three temperatures (288.15, 298.15, and 308.15) K were designed. The binodal data of the studied systems were determined and correlated with three experience equations, and a satisfactory correlation effect was gained. The effect of the type of salt on the phase-forming ability of the system was discussed in the study of effective excluded volume (EEV), phase diagram, and Gibbs free energy of ionic hydration. It was found that the phase-forming ability of the system was enhanced with the increase in system EEV and anion valence, and it also increased with an increase in the absolute value of Gibbs free energy of ionic hydration. The binodal curves are influenced by the system temperature, and it was found that, with the increase in temperature, the binodal curves moved to the left and the two-phase areas expanded. Moreover, the effect of temperature on the slope of the tie line was studied, and we have drawn a conclusion that the value of the slope of the tie line increased with rising temperature.