The　efficient　separation　of　Li+/Mg2+　is　the　key　to　lithium　extraction　from　salt　lake　brine.　Herein,　a　multi-amino　quaternary　ammonium　salt　(3,5-dimethylhydrazide-benzyl　trimethyl　ammonium　bromide,　DHTAB)　was　designed　and　synthesized　to　modify　pristine　polyamide　(PA)　nanofiltration　membranes.　DHTAB　was　grafted　on　the　surface　of　the　PA　nanofiltration　membrane　to　construct　a　positively　selective　layer　for　Li+/Mg2+　separation.　The　introduction　of　DHTAB　increased　the　membrane　surface　charge　density　to　+5.16　mC　m-　2,　which　was　nearly　three　times　that　of　the　pristine　PA　membrane.　The　increased　charge　density　of　the　modified　membrane　led　to　a　stronger　Donnan　exclusion　for　Mg2+,　and　accordingly　a　high　MgCl2　rejection　of　99.2%.　The　modified　membranes　exhibited　a　high　Li+/Mg2+　separation　factor　of　up　to　60.1,　when　applied　for　MgCl2　and　LiCl　binary　solution,　which　exceeded　all　known　PA-based　nanofiltration　membranes.