Electrochemical reduction of CO2 has been studied in 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BmimCF3SO3)/propylene carbonate (PC) solution on Au electrode. The linear sweep voltammetry measurements show that BmimCF3SO3 has significant catalytic effect toward CO2 reduction. In order to study the mechanism of CO2 reduction in BmimCF3SO3/PC on Au electrode, the electrochemical impedance spectroscopy is employed. The experiment results show that the electrochemical impedance spectroscopy detected in BmimCF3SO3/PC is different from that detected in tetrabutylammonium trifluoromethanesulfonate (Bu4NCF3SO3)/PC solution. The reasonable explanation is that Bmim+ is absorbed on the Au electrode, leading to the formation of ionic liquid film. During the CO2 reduction process, absorbed CO2 is reduced to CO2− radical via single electron transfer. The generated CO2− radical interacts with Bmim+ and inducing the formation of Bmim-CO2(ad). Thus, the activation energy and overpotential of CO2 reduction are reduced. The electrochemical impedance spectroscopy can be used as a methods to investigate the mechanism of CO2 reduction in non-aqueous solution.