The carbon-based interlayers set between the sulfur cathode and the separator have been demonstrated to be a simple but powerful means to promote the electrochemical performances of the Li-S batteries. However, the detailed mechanism has yet been fully understood. In this work, a series of battery configurations have been designed to further analyze the function mechanism of the carbon interlayer. The controlled studies demonstrate that the function of the interlayer as an accessorial current collector to provide and transport electrons plays the most important role in enhancing the dischargeability and reversibility of the sulfur cathode. The fundamental understanding of the function mechanism will provide further insight for the rational design of the interlayer structure in the Li-S battery system. (C) 2018 The Electrochemical Society.