Preparation of high-purity stable sulfur isotopes and their derivatives from the natural source is crucial to the research in the fields of pharmacology, ecology, and geology. Nevertheless, conventional isotope separation methods usually involve a complex, hundreds-of-steps cascade separation procedure and, therefore, high costs on materials, facilities, and labor. In this work, we developed, based on the knowledge about diffusion kinetics of isotopic-S-based polysulfides in a lithium–sulfur (Li-S) battery, an electrochemical method for separating stable ³²S/³⁴S isotopes in a scalable and cascadable electrolysis cell. Experimental data have revealed that the distance between 2 electrodes correlates positively with the separation factor and negatively with the yield. An appropriate interelectrode distance helps to balance separation factor and yield to achieve the optimal separation effect. Based on the optimized parameters, we demonstrated the preparation of high-purity ³²S/³⁴S isotopes from their natural combination via <15 cascade separation steps, showing a prominent advantage over conventional methods.