In situ polymerization of acrylonitrile with nano-sulfur particles has been developed as a synthetic route to prepare sulfur/polyacrylonitrile (S/ p PAN) composite as a cathode material for lithium/sulfur battery. Transmission electronic microscopy revealed the formation of a highly developed network structure consisting of PAN and sulfur homogeneous mixing at nanosized level, providing the “buffering” space to accommodate the volume change of sulfur upon cycling and retaining the structural integrity preventing the material agglomeration and degradation. Benefiting from this unique structure, the S/ p PAN composite cathode demonstrated enhanced reversibility, resulting in a discharge capacity of 1177 mAh g−1 at the second cycle and retained about 100% of this value over 100 cycles at 0.5C. Furthermore, the S/ p PAN composite cathode delivered a discharge capacity of 981 mAh g−1 at the 100th cycle at 1C.