While concentrated seawater discharged by desalination plants requires considerable efforts to alleviate undesired environmental risk, the conversion of salinity gradient energy to electric energy is considered an ideal way. In this work, the experiment and simulation of multi-stage reverse electrodialysis (MS-RED) were carried out, and the influence of flow rate and compartment thickness of high concentration compartment (HCC) and low concentration compartment (LCC) on the MS-RED performance were investigated independently for the huge concentration difference between HCC and LCC. The optimized flow rates of LCC and HCC were 0.96 cm·s−1 and 0.71 cm·s−1, and the compartment thicknesses of LCC and HCC were 1.05 mm and 0.85 mm, respectively. Furthermore, the performance of series multi-stage reverse electrodialysis (SMS-RED) and independent multi-stage reverse electrodialysis (IMS-RED) was compared, and SMS-RED showed better performance than IMS-RED. With the optimized conditions of SMS-RED, the power generation and energy efficiency were reached 684.07 W and 16.22 % after 20 stages, respectively, while the concentration of concentrated seawater was reduce from 66.70 to 37.63 g·L−1. The results can provide a theoretical basis for the industrial development of MS-RED to achieve the deep utilization of concentrated seawater and reduce the environmental risks of its direct discharge.