Solid-liquid phase equilibrium information is essential to the research and development of crystallization processes. Computer simulation of the multicomponent solid-liquid equilibrium avoids the traditional tedious experimental determination;The phase equilibrium simulation requires an accurate thermodynamic model to describe the solution chemistry and a usable mathematical procedure to obtain reliable solutions: In this work, a modified activity coefficient model is presented. The modification makes the model more practical to use. A new numerical algorithm, which is based on a large-scale optimization technique is used for phase equilibrium calculation. This new method takes advantage of the thermodynamic properties of the solid-liquid equilibrium and unifies thermodynamics and Mathematics; The numerical procedures have real physical meanings. The phase diagram at various temperatures of the industrial important system Na-K-Mg-Cl-NO[subscript]3- H[subscript]2O is calculated using the new method. The results compare well with the available experimental data.