Lithium–sulfur (Li–S) batteries are being explored as promising advanced energy storage systems due to their ultra-high energy density. However, fast capacity fading and low coulombic efficiency, resulting from the dissolution of polysulfides, remain a serious challenge. Compared to weak physical adsorptions or barriers, chemical confinement based on strong chemical interaction is a more effective approach to address the shuttle issue. Herein, we devise a novel polymeric sulfur/carbon nanotube composite for Li–S battery, for which 2, 5-dithiobiurea is chosen as the stabilizer of long-chain sulfur. This offers chemical bonds which bridge the polymeric sulfur and carbon nanotubes. The obtained composite can deliver an ultra-high reversible capacity of 1076.2 mAh g−1 (based on the entire composite) at the rate of 0.1 C with an exceptional initial Coulombic efficiency of 96.2%, as well as remarkable cycle performance. This performance is mainly attributed to the reaction reversibility of the obtained polymeric sulfur-based composite during the discharge/charge process. This was confirmed by density functional theory calculations for the first time.