Abstract: The complexation process of uranyl(Ⅵ) with trioctylphosphine oxide (TOPO) in trimethyl-butylammonium bis (trifluoromethylsulfonyl) imide (N₄₁₁₁NTf₂) was investigated by cyclic voltammetry and IR and UV-vis spectrometry. The complex shows a vibronic progression with an average interval of (728±9)cm^-1 in its UV-vis spectrum. While the asymmetric stretching vibration frequency (ν_as)  of the complex is 926cm^-1 in its infrared spectrum. Two successive one-electon tranfer steps take place in the electroreduction process of the ［UO₂(TOPO)₄］²⁺, with the corresponding reduction peak potentials of E_pc1=-1.37V and E_pc2=-1.75V （vs. Ag⁺/Ag）. The first electron transfer step is quasi-reversible while the latter one is irreversible. The half wave potential of the quasi-reversible process of U(Ⅵ)→U(Ⅴ) is -1.33V, which is stable with respect to changes in scan speed and temperature. That implies the stablity of the process and the reduction product. The activation energy of the U(Ⅵ)→U(Ⅴ) reaction is calculated as (39.0±1.1)kJ/mol. The system may possess potential application prospect in the electroseparation process with ionic liquid.