Lithium-sulfur　batteries　have　become　an　appealing　candidate　for　low-cost　and　high-energy-density　power　sources.　It　is　generally　believed　that　the　shuttle　effect　in　Li/S　batteries　causes　serious　performance　degradation,　such　as　low　coulombic　efficiency,　sulfur　loss,　and　so　on.　Here,　we　report　a　new　mechanism　based　on　the　disproportionation　reactions　of　polysulfides　(PSs)　to　explain　the　rapid　fading　of　capacity　in　Li/S　batteries.　Two　different　electrolytes　(i.e.,　a　carbonate-based　electrolyte　and　an　ether-based　electrolyte)　were　adopted　to　study　the　disproportionation　mechanism.　We　found　that　elemental　sulfur　was　reprecipitated　on　the　surface　of　the　cathodes,　forming　a　passive　layer.　The　passive　layer　formed　in　the　carbonate-based　electrolyte　is　much　denser　than　that　formed　in　the　ether-based　electrolyte,　resulting　in　the　faster　capacity　fading　in　the　carbonate-based　electrolyte.　The　simulation　of　cathodic　peaks　and　anodic　peaks　in　CV　profiles　confirms　the　disproportionation　reaction　of　PSs　in　the　ether-based　electrolyte.　The　electrochemical　behavior　of　these　Li/S　batteries　are　well　explained　by　an　EC　mechanism　that　involves　an　electron　transfer　reaction　followed　by　a　reversible　disproportionation　reaction.　It　is　found　that　the　disproportionation　of　PSs　plays　a　more　important　role　than　the　shuttle　effect　during　the　initial　stage　of　the　charge/discharge　process.