Abstract: The hydrophobic 1-alkyl-3-acetyl-benzo21 crown 7 bis(trifluoromethylsulfonyl)imide ionic liquidC_n(benzo21C7)AImNTf₂(n=4,6,8) abbreviated by C_n-CrownIL were synthesized. The synthesized crown ether ionic liquids were characterized by IR and NMR. The crown ether ionic liquid was mixed with 1-butyl-3-methyl-imidazole bis(trifluoromethylsulfonyl)imide ionic liquid (C₄MImNTf₂) to construct C_n-CrownIL/C₄MImNTf₂ complexes. It was found that the hydrophobicity of C_n-CrownIL/C₄MImNTf₂ ensured the formation of a phase separation system of aqueous solution. The results showed that C_n-CrownIL/C₄MImNTf₂ had a high extraction for Rb⁺ and Cs⁺, the extraction efficiency was almost 100% which was higher than other metal ions, and the order of metal ions extraction efficiency was Cs⁺ > Rb⁺ > K⁺ > Ca²⁺ > Na⁺ > Mg²⁺ > Li⁺. Even though the concentrations of Rb⁺ and Cs⁺ were as low as 10 mg/L, the extraction efficiency remained above 97%. The addition ofC₄MImNTf₂ in the extractant greatly improved the extraction efficiency. The extraction rate of C_n-CrownIL/C₄MImNTf₂ was very high and the extraction equilibrium could be achieved after 10 min. By increasing of alkyl chain length, the extraction efficiency of NTf₂^- crown ether for Rb⁺ and Cs⁺ decreased slightly, but still was maintained above 85%. The extraction efficiency of Rb⁺ and Cs⁺ increased with the increase of solution pH. The recycle of C_n-CrownIL/C₄MImNTf₂ can be realized by adding diluted nitric acid to reextract Rb⁺ and Cs⁺. Results from recycling tests showed that the extraction efficiency of Rb⁺ and Cs⁺ was still above 90% after five recycling tests. Theoretical calculations showed that the extraction of C₄-CrownIL/C₄MImNTf₂ for Rb⁺ and Cs⁺ depended on the strong complexation of the oxygen atom in the crown ether with Rb⁺ and Cs⁺, but the primary reason was that the cavity size of crown ether highly matched the ion-radius of Rb⁺ and Cs⁺.