Abstract: By using high isotope effect extracting agent benzo-15-crown-5(B15C5) and green and stable solvent—ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide-［C₄MIm］NTf₂),  liquidliquid extraction system B15C5--［C₄MIm］NTf₂/LiA(anions were  bis(trifluoromethylsulfonyl) imide(NTf^-₂), trifluoromethanesulfonate(OTf^-) and Cl^-)-H₂O was established to separate lithium isotopes efficiently. Equilibrium time, effect of concentration of B15C5 and lithium salts, temperature and pH on distribution coefficient(D) were examined. The results indicate: the extraction equilibrium are short, t≤ 30 s; increasing the concentration of B15C5 and reducing the concentration of lithium salts are conducive to improve the distribution coefficient(D). Under the same conditions, the order of the D is LiNTf₂＞LiOTf＞LiCl, much higher than other extraction system; temperature and pH have no significant effect on the distribution coefficient. In addition, single crystal structure analysis of the complexes shows that all lithium salts are formed the same 1∶1 complex as ［Li₄(B15C5)₄(OH)₂(H₂O)］(NTf₂)₂ with B15C5, indicating that Li⁺ is most likely extracted into organic phase by cation exchange process in ionic liquid system. The extraction system is efficient, green and has large capacity of lithium, it may have a high application value in industrial production.