Salinity gradient energy extracting by a reverse electrodialysis (RED) unit using for electrodialysis (ED) desalination process is a potential way to achieve energy-economic and sustainable production of freshwater. However, the parameters in RED and ED unit synergistically influence the desalination process, resulting to the hybrid process controlled by multi-parameters. Modeling of an RED/ED is a simple way to describe the desalination process and reveal the effects of these parameters on the performance of system and then to find the better adaption of RED/ED. In this study, a model of an internal-integrated RED/ED hybrid system is first established. It found that the ratio of desalination in RED/ED is higher than 90%. The brine/river is the alternative combination to realize seawater desalination with a desalination rate of 0.38 h m2/mol. The desalination capacity of RED/ED (0.43–2.6 mol/h·m2) is much higher than that of the external-integrated RED + ED system (0.10–0.15 mol/h·m2), but it is of simpler configuration and has a lower energy requirement. Moreover, the RED/ED system is preferred for using in the pre-desalination process. The outcome of this model is helpful in the design of practical RED/ED systems, and points out the development potential of RED/ED in practical applications.