As　an　important　resource,　lithium　is　abundant　in　Salt　Lake.　The　high　Mg2+/Li+　ratio　is　the　main　obstacle　to　lithium　extraction.　Positively-charged　nanofiltration　membrane　has　shown　great　potential　in　the　separation　of　Mg2+/Li+.　Herein,　we　report　a　facile　method　to　construct　positively-charged　nanofiltration　membrane　by　polyethyleneimine　(PEI)　and　layered　double　hydroxide　(LDH)　nanosheets　on　hydrolyzed　polyacrylonitrile　ultrafiltration　membrane.　Subsequently,　the　PEI-LDH　composite　membrane　was　crosslinked　with　glutaraldehyde　(GA)　to　improve　its　stability.　The　incorporation　of　LDH　could　improve　the　positive　charge　of　the　membrane.　Then,　the　obtained　PEI-LDH/GA　membrane　showed　an　enhanced　separation　performance　for　separating　Mg2+/Li+　in　an　aqueous　solution.　When　the　Mg2+/Li+　ratio　was　10,　the　membrane　showed　a　separation　factor　(SMg2+/Li+)　of　18.7.　Moreover,　the　membrane　could　maintain　the　separation　performance　for　100　h,　which　exhibited　a　potential　application　in　Mg2+/Li+　separation.