Lithium-sulfur batteries have attracted a great interest in electrochemical energy conversion and storage, but their discharge mechanism remains not well understood up to now. Here, we report density functional theory (DFT) calculation study of the discharge mechanism for lithium-sulfur batteries which are based on the structure of S8 and Li2S x (1≤x≤8) clusters. The results show that for Li2S x (1≤x≤8) clusters, the most stable geometry is chainlike when x=1 and 6, while the minimal-energy structure is found to be cyclic when x = 2–5, 7, 8. The stability of Li2S x (1≤x≤8) clusters increases with the decreasing x value, indicating a favorable thermodynamic tendency of transition from S8 to Li2S. A three-step reaction route has been proposed during the discharge process, that is, S8→Li2S4 at about 2.30 V, Li2S4→Li2S2 at around 2.22 V, and Li2S2→Li2S at 2.18 V. Furthermore, the effect of the electrolyte on the potential platform has been also investigated. The discharge potential is found to increase with the decrease of dielectric constant of the electrolyte. The computational results could provide insights into further understanding the discharge mechanism of lithium-sulfur batteries.