The polymer polyoxyethylene(20) cetyl ether (C 56 H 114 O 21 , POELE20) and three salts (K 2 C 2 O 4 , K 2 C 4 H 4 O 6 , and K 3 C 6 H 5 O 7 ) were the phase-forming materials to build the aqueous two-phase system (ATPS) at the temperatures 288.15, 298.15, and 308.15 K. The data of the phase diagram including binodal curve data and tie-line data were determined and fitted by classical equations, and they obtained the satisfactory correlation effect. The phase-forming ability of salt composed of the same cation was investigated by three factors, namely, the valence of the anion, the Gibbs free energy of the anion, and the value of effective excluded volume (EEV). It was found that the phase-forming ability of salt strengthened when the values of said three factors went up. The results of the analysis on the data from the present work and that in our previous paper showed that the order of the phase-forming ability of salts was KOH