In this study the vapor pressures of three mixed electrolyte aqueous solutions, NaCl-KCl-H2O, NaBr-KBr-H2O and NaCl-CaCl2-H2O, have been measured in the temperature range 298-343 K by the static method. The maximum ionic strength of the three solutions under study is 4 m. A primary model based on the mean spherical approximation principle is proposed to predict the vapor pressure of mixed electrolyte solutions. The parameters used in this model are obtained directly from the results of single electrolyte solutions reported in our previous work. No additional parameters are needed. The predicted results agree well with the experimental ones, with the overall mean deviations being 0.70, 1.02 and 0.64% for the three solutions respectively, The mean deviations for concentrated solutions with ionic strengths higher than 3 m are much larger than those for dilute solutions, In order to obtain more accurate results, a modified model is proposed by introducing an adjustable parameter to account for the interactions between the mixed electrolyte ions, The correlated results by the modified model are satisfactory. The mean deviations between the calculated and experimental results for concentrated solutions are largely reduced, and the overall mean deviations are 0.42, 0.71 and 0.47% for the three solutions respectively.