Polysulfide (PS) shuttle, which is one of the critical problems that need to be addressed for realizing lithium sulfur batteries, can be suppressed by confining PSs within microporous or mesoporous templates. However, PS dissolution through the external opening of the templates has to be further prevented. In this work, a poly (3,4-ethylenedioxythiophene) (PEDOT) layer is formed in situ on the CMK-3/S cathode by inducing the electro-oxidative polymerization of the EDOT monomer included in electrolytes during pre-charging. The PEDOT layer covering the external cathode surface functions as a PS blocking layer, which suppresses the PS shuttle and, thus, improves the cycling stability. The spectroscopic analysis indicates that the PEDOT layer reduces the decomposition of the electrolyte by preventing any reaction between the active PS species and electrolytes. Therefore, the in situ formation of the PS blocking layer from the electrolyte additive provides a simple and effective method to improve the cycling stability of lithium sulfur batteries.