Organic polysulfanes are new type of attractive organosulfur electrode materials for next generation lithium-sulfur (Li-S) batteries because of their high sulfur content, low cost, and desirable energy density. However, conventional organic polysulfanes usually suffer from poor reversibility due to structure variation and irreversible conversion during cycling. Here we report the synthesis and characterization of a novel two-dimensional (2D) organic polysulfane with a unique molecular structure of polycyclic sulfur directly substituting the carboxyls of poly(acrylic acid) and grafted on the carbon chain through a coupling reaction with KI as a catalyst and KCl as a template. The obtained organic polysulfane nanosheets with 72 wt% sulfur (OPNS-72) exhibit high initial capacity of 891 mAh/g (based on whole composite), excellent cycling stability (0.014% capacity fading per cycle over 620 cycles at 1 C rate), superior rate capability (562 mAh/g at 10 C) and high mass loading of 9.7 mg/cm2. The remarkable cycling stability of the Li-S battery is attributed to the structural stability and highly reversible electrochemical reaction of the OPNS-72 electrode, as confirmed by the TEM image after cycling and operando Raman spectroscopy measurements under battery operating conditions. Further, the developed synthesis approach is applicable for the preparation of other organic polysulfane nanosheets as highly reversible electrodes for Li-S batteries.