Disclosed herein is an iodine-sulfur cycle for nuclear hydrogen production, which can improve thermochemical efficiency. The iodine-sulfur cycle is advantageous in that the amount of excess water and iodine supplied to a Bunsen reaction process is minimized, thus minimizing the amount of thermal energy consumed in the recovery and recirculation process thereof, in that sulfuric acid having stronger hydrophilicity than hydrogen iodide absorbs excess water in large quantities in a spontaneous liquid-liquid phase separation process, so that, after the spontaneous liquid-liquid phase separation process, the concentration of hydrogen iodide in a hydrogen iodide solution exceeds a concentration at an azeotropic point without conducting an additional concentration process, with the result that highly-concentrated hydrogen iodide gas can be obtained only through a flashing process, thereby decreasing energy consumption and simplifying the process and thus improving economical efficiency, and in that process temperature and pressure can be decreased, thus greatly deceasing the corrosivity in an operational environment. Therefore, the iodine-sulfur cycle according to the present invention can be usefully used for high-efficient and environmentally-friendly nuclear hydrogen production.