To understand the mechanism for preventing the shuttle in lithium–sulfur batteries, the SEI films formed in different electrolyte solutions are studied using X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and electrochemical measurements. The SEI film formed in electrolyte solution with LiNO3, which is an effective additive to suppress the shuttle, cannot maintain a stable state without polysulfides. The combination of electrochemical data and SEM images indicate that the contribution of LiNO3 and polysulfides are equally important to form a SEI film which is effective in suppressing the shuttle. This SEI film can be assumed to consist of two sub layers, the top layer composed of oxidized products from polysulfides (lithium sulfates) and the bottom layer composed of the reduced products of polysulfides and LiNO3 (lithium sulfide and LiN x O y ). We suppose the formation process of this SEI film in detail and give a possible explanation of the mechanism by which the SEI film on the lithium electrode is preventing the shuttle in lithium–sulfur batteries.