Binodal data for the ethanol/1-propanol/2-propanol + ammonium sulfate + water systems were determined at 298.15 K. On the basis of ail empirical equation and lever rule, the liquid-liquid equilibrium compositions of the ethanol + ammonium sulfate + water system were directly calculated by Matlab. The Othmer-Tobias equation and Bancroft equation proved the reliability of the calculation method and the corresponding tie-line data. The phase-separation abilities of hydrophilic alcohols were compared by the effective excluded volume (EEV) of salt and the binodal curves plotted in molality. The phase-separation abilities of the investigated alcohols are in the order of I-propanol > 2-propanol > ethanol > methanol. The mechanism of phase separation and the effects of alcohols, salts, and pH values are also discussed. The results show that the increase in the amount of phase-forming substance and the decrease in pH values (from 9.03 to 3.58) are both advantageous to the formation of the aqueous two-phase system (ATPS).