Reverse electrodialysis (RED) is the most promising technique based on ion exchange membranes for harvesting salinity gradient energy. In the present study aimed to explore the sensitivity of the RED stack performance, different concentrations of NaCl were prepared and used to simulate the salinity of concentrated seawater and that of river water at 298 K. Some major factors including the repeating unit pairs, the flow velocity and the feeding concentration were investigated. When the positive effect by the increase of open-circuit voltage is greater than the negative effect caused by the increase of internal resistance, the electrical performance of RED stack is improved, and the maximum power density is increased. On the contrary, the value of maximum power density is decreased. The maximum power density reached its maximum when eight repeating units were used, and the optimum flow velocity was obtained as 0.71 cm/s. At a same concentration gradient of the feed, a lower internal resistance was observed at a higher total salinity, leading to better energy generation performance.