In this study, a new CO 2 electroreduction electrolyte system consisting of tetrabutylphosphonium 4-(methoxycarbonyl) phenol ([P 4444 ][4-MF-PhO]) ionic liquid (IL) and acetonitrile (AcN) was designed to produce oxalate, and the electroreduction mechanism was studied. The results show that using the new IL-based electrolyte, the electroreduction system exhibits 93.8% Faradaic efficiency and 12.6 mA cm À2 partial current density of oxalate at À2.6 V. The formation rate of oxalate is 234.4 μmol cm À2 h À1 , which is better than those reported in the literature. The mechanism study using density functional theory (DFT) calculations reveals that [P 4444 ][4-MF-PhO] can effectively activate CO 2 molecule through ester and phenoxy double active sites. In addition, in the phosphonium-based ionic environment, the potential barriers of the key intermediates *CO 2 À and *C 2 O 4 2À are reduced by the induced electric field, which greatly facilitates the activation and conversion of CO 2 molecule to oxalate.