This　study　first　systematically　investigated　selective　Li+　extraction　from　Mg2+-rich　brines　with　Na+　and　K+　using　ionic　liquid　(IL)　based　synergistic　extractant　(SE)　systems　from　molecular　mechanisms　to　extraction　performances.　The　ternary　combination　1-allyl-3-methylimidazolium　bis(trifluoromethanesulfonyl)imide　([AMIM]　[Tf2N]),　tri-iso-butyl　phosphate　(TIBP)　and　dichloromethane　(DCM),　i.e.,　[AMIM][Tf2N]-TIBP-DCM　was　selected　as　a　suitable　SE　from　the　diverse　organic　phosphorus　ligands,　ILs　and　diluents.　The　single-stage　Li+　extraction　efficiency　was　as　high　as　96.87%,　and　selectivities　of　beta　Li+/Mg2+,　beta　Li+/Na+,　and　beta　Li+/K+were　up　to　1161.94,　76.74,　and　1263.46,　respectively.　It　was　found　that　Li+　is　extracted　from　the　aqueous　phase　to　organic　phase　in　Li+-4TIBP[Tf2N]　complex.　The　molecular-level　mechanism　was　identified　as　the　multi-type　interaction　formed　between　metal　ions　and　[AMIM][Tf2N]-TIBP　to　destroy　the　hydration　of　metal　ions　through　quantum　chemical　(QC)　calculations.　The　work　proposes　valuable　theoretical　guidance　to　develop　novel　IL-related　SE　systems　for　the　high-efficiency　Li+　extraction　from　salt　lake　brines.