Highly efficient separation of aromatics and aliphatics mixtures is important in the chemical industry. In this work, we designed several new composite extractants formed by a dicationic ionic liquid (DIL, [C5(MIM)2][NTf2]2) and solvents, which have low viscosity and could enhance the mass transfer in the extraction process. We systematically investigated and compared the viscosity and separation performance between the pure DIL and composite extractants. The results indicated that the type of solvents and their content in composite extractants had a big influence on separation performance. Among the composite extractants, DIL-TTEG (tetraethylene glycol) showed the best extraction performance. The highest selectivity (S) and distribution (Dtol) for toluene are 38.8 and 1.05, respectively, at a mass fraction of toluene = 0.1, mDIL:mTTEG = 10:1, and T = 30 °C. At these conditions, compared to the DIL, the viscosity of DIL-TTEG decreases 43.0%, while, the S and Dtol still keep high values. Notably, the S and Dtol are 1.89 and 2.56 times of pure TTEG. The best performance for benzene/n-hexane and benzene/cyclohexane systems are S: 46.8 and 23.7, D: 1.92 and 1.78, respectively, at 0.1 mass fraction of benzene. The results show that DIL-TTEG not only has relatively low viscosity, but also shows good extraction performance. Furthermore, it can be regenerated simply by distillation. Given these properties, DIL-TTEG is a promising composite extractant to selective extraction aromatics from aliphatics in the industry.