When evaluating ionic liquids(ILs) for extractive desulfurization(EDS) of fuel oils, the inevitable presence of water in the system may have a significant and in many cases strongly negative effect. However, few studies have considered this particular issue and a promoted water effect on EDS is scarcely reported. In this work,COSMO-RS was firstly employed to calculate the capacity and selectivity for EDS of various IL/H_2 O mixtures,which cover different IL characters and a wide water concentration range. Experiments were then conducted with a representative IL [C_4MIM][H_2PO_4], whose stable and even promoted extraction performance with a small amount of water was suggested by COSMO-RS. Through analyses of the desulfurization ratio, the crosssolubility and the water content in the desulfurized fuel, the promoted effect of water within a certain range(b 10 wt%) was experimentally demonstrated. Moreover, such effect of water was explained combining the viscosity, the solvent–solute interactions and the COSMO-RS based analysis. However, few studies have considered this particular issue and a promoted water. (However, few studies have considered this particular issue and a promoted water effect on EDS is scarcely reported. In this work, COSMO-RS was firstly employed to calculate the capacity and selectivity for EDS of various IL / H_2 O mixtures, which cover different IL characters and a wide water concentration range. Experiments were then conducted with a representative IL [C_4MIM] [H_2PO_4], whose stable and even promoted extraction performance with a small amount of water was suggested by COSMO-RS. Through analyzes of the desulfurization ratio, the crosssolubility and the water content in the desulfurized fuel, promoted effect of water within a certain range (b 10 wt%) was experimentally demonstrated..., such effect of water was explained combining the viscosity, the solvent-solute in teractions and the COSMO-RS based analysis.