Electrodialysis (ED) was used to treat a saline water with a high Mg/Li ratio. The main objective of this study was to significantly reduce the Mg/Li ratio. The Mg/Li ratio can be decreased as high as 21.8 times via ED, and the Li+ recovery can reach over 90%. ED was proven to be technically and economically feasible to separate lithium from salt lake brine with high Mg/Li ratio. The selective separation mechanism of ED was qualitatively discussed.