The effect of ionic liquids C(n)mimBr (n = 4, 6, 8) on the properties of cationic Gemini surfactant trimethylene-1,3-bis(dodecyl ammonium bromide) (Gemini 12-3-12) aqueous solution was investigated. The critical micelle concentration (cmc) of Gemini 12-3-12 solutions containing different amounts of ionic liquids were obtained by measurement of surface tension. The composition of mixed micelle and the interaction parameter between ionic liquids and Gemini 12-3-12 were calculated by Rubingh regular solution model; the effective area of Gemini 12-3-12 molecule at air/water interface was calculated by Gibbs adsorption isotherm. The results show that the addition of ionic liquid has important effect on the properties of surfactant aqueous solution. The cmc of surfactant solution and the effective area of surfactant molecules at the air/water interface vary parabolically with the addition of ionic liquids owing to the two extra opposite effects of ionic liquids in surfactant aqueous solutions besides counterion effect, participation of micelles formation, and the effect on the properties of solvent. The effect of ionic liquids on surfactant solution can be regarded as the comprehensive effects of inorganic salt, cosurfactant, and cosolvent on surfactant solution.