Lithium sulfur (Li-S) batteries are very promising electrochemical storage system due to their high gravimetric energy density and low cost. Enormous efforts have been put on Li-S battery to achieve its commercialization. The function of electrolyte is a key issue in achieving the high performance of Li-S system, and several additives have been tried. But very few works have been working on the electrolyte optimization method with the consideration of over-all performance, including cycling stability, rate capability, especially self-discharge prevention ability, and the consideration of loading condition. In this work, we focus on the incorporation of room temperature ionic liquid (IL) as co-solvent, and the effect of IL in mitigating the polysulfide dissolution via systematical mechanism study of self-discharge phenomena. Moreover, the optimization of IL incorporation ratio is discussed for the sake of the better over-all electrochemical performance of Li-S cell by considering cycling stability, rate performance and self-discharge prevention ability, which is found to vary with loading condition. The improved understanding of the effect of IL on battery performance will help the development of electrolyte for Li-S batteries.