In this paper, the thermodynamic properties, such as the mean activity coefficient, osmotic coefficient, and water activity, for 129 complex aqueous electrolyte solutions have been calculated by the modified three-characteristic-parameter correlation model. We also calculated the adjustable parameters of the Pitzer model and the Bromley model. Results of the standard deviations showed that our modified model is fairly suitable for predicting the mean activity coefficient, osmotic coefficient, and water activity of these electrolytes. This new model is much better than the Bromley model for all studied electrolyte solutions in this paper, and it is almost on par with the Pitzer model. Especially for 3-1 and 3-2 electrolytes, our model shows a better result than the Pitzer model.