The solubility of the KCl-K2SO4-H2O system was simulated at a wide range of temperatures by using PSC model and Pitzer model. The experiment water activity and experiment solubility in the binary KCl-H2O, K2SO4-H2O and ternary KCl-K2SO4-H2O systems were used to obtain the model parameters. For the KCl-K2SO4-H2O system of low solubility, the Pitzer model values are in agreement with the experiment data just by using binary coefficients, and the PSC model should take into account the ternary coefficients.