The commercialization of lithium–sulfur batteries is hindered by serious capacity fading that mainly results from the irreversible oxidation of sulfur active material during cycling, but so far the underlying oxidation mechanism still remains unclear. The research results reveal that the most practical solvent DOL and DME are not stable and large amount of degradation products with -OLi edge groups become the oxygen source. The shuttle phenomenon is an important reason leading to the irreversible oxidation of sulfur active material, which results from series chemical reductions of lithium polysulfide, and they keep charge conservation by their selves, but prolong the charge process and lead to the overcharging. In charge process, the low speed reactions of long-chain lithium polysulfide oxidized to Li x SO y species exist in the electrolyte, which are indistinctly in the systems without overcharging. As the shuttle phenomenon prolongs the charge process, the irreversible oxidation reactions become distinctly, and the Li x SO y species are detectable. The overcharging capacity could be explained as a reversible part and an irreversible part. The reversible capacity is from the cathode active material oxidized to long-chain lithium polysulfide and elemental sulfur, and the irreversible capacity is from the long-chain lithium polysulfide oxidized to Li x SO y species.