Abstract: The tail solution after lithium extraction from salt lake still contains a large amount of lithium，and directly disch arging to the salt field is not only causing waste of lithium resources，but also damaging the salt lake system. Reducing the ra tio of Mg2+ to Li+ in the tail solution is the key to recovery of lithium in the tail solution.Nanofiltration and reverse osmosis combined technonlogy is successfully used to recovery the lithium in the lithium extraced tail solution.The separation effect of 7 types nanofiltration membranes on Mg2+ and Li+ in the tail solution was investigated，the results showed that 1# nanofiltration membrane has the best separation effect.Take 1# nanofiltration membrane as the nanofiltration element，the separation effect of Mg2+ and Li+ in the tail liquor was investigated under different filtration pressure，temperature and dilution ratio of the tail solution.The optimal operating conditions were as follows：the filtration pressure was 4 MPa，the dilution ratio was 6 times and the temperature was 35 ℃.The lithium extracted tail solution was separated by twostage nanofiltration under optimal operating conditions via No.1 nanofiltration membrane，then the lithium rich liquid phase was concentrated by reverse osmosis to obtain lithium riched solution with the mass ratio of Mg2+ to Li+ was 13.8 and the mass concentration of Li+ was 0.39 g/L. Lithium riched solution can be used to produce battery grade lithium carbonate by reacting with soda after concentrated and impurity removal.The liquid phase with high Mg2+ content retained by nanofiltration was discharged to the tail solution pool，then was evaporated and concentrated，and was recoverd and reused after discharging into the salt field.

Key words: lithium extracted tail solution；nanofiltration；reverse osmosis；ratio of magnesium to lithium；Li2CO3