One of the challengeable works for the Iodine-Sulfur water splitting cycle is to establish the thermodynamic model for HI section due to the difficulty of the modeling of reactive distillation column and non-ideal characteristic of HIx solution. In order to predict partial pressure of HIx solution in the binary mixture (H2O-HI), the thermodynamic model for HIx solution need to be developed with a proper binary parameter set. In this study, Neumann’s thermodynamic NRTL model for the HIx solution was reproduced and KAIST model (improved Neumann model) was proposed with new assumptions. The binary parameter set for binary mixture and ternary mixture was regressed by using the numerical muti-variable fitting method in MATLAB. We validated the KAIST model with the new binary parameter set through comparison with experimental data. Besides, we proposed the simple Liquid-Liquid Equilibrium (LLE) phase separation method. The proposed method can predict LLE separation around 30%~34% of HI concentration with only total pressure calculation based on the KAIST model. In ternary mixture(H2O-HI- I2), taking the same procedure as in the binary mixture we get the new parameter set to better predict the partial pressure of HIx solution considering HI decomposition effect. We developed the two-step method for the hydrogen partial pressure calculation. We compared hydrogen partial pressures obtained by the KAIST model with those by the Neumann model.