Lithium-sulfur batteries possess favorable potential for energy-storage applications because of their high specific capacity and the low cost of sulfur. Intensive understanding of the interfacial mechanism, especially the polysulfide formation and transformation under complex electrochemical environment, is crucial for the buildup of advanced batteries. Here, we report the direct visualization of interfacial evolution and dynamic transformation of the sulfides mediated by the lithium salts via real-time atomic force microscopy monitoring inside a working battery. The observations indicate that the lithium salts influence the structures and processes of sulfide deposition/decomposition during discharge/charge. Moreover, the distinct ion interaction and the diffusion in electrolytes manipulate the interfacial reactions determining the kinetics of the sulfide transformation. Our findings provide deep insights into surface dynamics of lithium-sulfur reactions revealing the salt-mediated mechanisms at nanoscale, which contribute to the profound understanding of the interfacial processes for the optimized design of lithium-sulfur batteries.