Direct discharge of seawater with high salinity and temperature from thermal desalination plants can cause marine ecological damage and energy waste. Here, the reverse electrodialysis (RED) approach is introduced to capture the salinity gradient energy (SGE) between concentrated seawater and seawater. It not only harvests the SGE and low-grade waste heat in desalination plants for power production, but also reduces discharge salinity of concentrated seawater. Firstly, the mass transfer in a single-stage RED stack is modeled and verified by experiments. Furthermore, the atlases of the performance evaluation indexes for the RED stack are drawn and analyzed. Finally, the multi-stage RED (MS-RED) stacks with independent circuit control strategy is proposed to harvest more SGE and make energy conversion more effective. Meanwhile, the variation law of performances of MS-RED with series is analyzed. For a single-stage RED stack with 10 pairs of membrane cells, its power density can reach 0.374 W/m2, and open-circuit voltage of 0.59 V. Appropriately increasing series of MS-RED stacks can improve the energy efficiency of converting more SGE into electric energy and obtain larger output power. This study can provide a certain guidance for the capture of SGE in thermal desalination plants for power production.