For the purpose of designing ionic liquid (IL) solvents for the extractive separation of benzene/cyclohexane at 298.15 K, a computer-aided ionic liquid design (CAILD) method is developed. The UNIFAC-IL model is used to calculate the thermodynamic properties, while the group contribution (GC)-based methods are employed to estimate the physical properties and toxicity. A mixed-integer nonlinear programming (MINLP) problem is formulated, and the top five IL solvents are obtained by the BONMIN algorithm. One of the designed ILs [COC2MIM][Tf2N] (1-(2-methoxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) is selected to perform the liquid-liquid extraction (LLE) experiment, and the distribution ratio and selectivity at 0.1 molar concentration of benzene in the raffinate phase are 1.20 and 17.10, respectively. The comparison with other solvents shows that the designed IL not only has an excellent separation performance but also favorable physical and environmental properties. After regressing the parameters for the NRTL model, the process simulation using the designed IL is developed by Aspen Plus, and the results are compared with that of the benchmark organic solvent sulfolane. The IL-based process needs 32475.1 kg/h of solvent and 1283 kW of heat duty, while the sulfolane-based process uses 43998.5 kg/h of solvent and 6296 kW of heat duty. These results demonstrating [COC2MIM][Tf2N] is a promising alternative to conventional solvents for the extractive separation of benzene/cyclohexane.