Lithium Sulfur battery is supposed to be one of the most promising candidates in next generation rechargeable batteries. Owing to the high theoretical specific capacity (~1675 mAh/g) and abundance of sulfur, lithium-sulfur battery can achieve high energy density with relatively low cost. However, the practical energy density is much smaller than the theoretical value. The operation of Li-S batteries under lean electrolyte condition was revealed by investigating Li-S pouch cell under different electrolyte/sulfur ratio. The cathode interfacial resistance was figured out in testing Electrochemical Impedance Spectroscopy (EIS) during discharging for different E/S ratio. The electrolyte resistance, interfacial resistance as well as the charge transfer resistance were keeping changing in same trend for different E/S ratio. The changing was observed in both DOL/DME and DMSO electrolytes for all E/S ratios we tested. Such resistance behaviors were also confirmed by enforced Galvanostatic Intermittent Titration Technique (GITT). By applying enforced GITT onto the lean electrolyte cell, it can even get as high specific capacity as flood electrolyte cells did. We propose that the cathode interfacial resistance can be one of the reasons which is challenging the Li-S battery to achieve its theoretical specific capacity under lean electrolyte condition. Such mechanistic understanding of the Li-S battery under lean electrolyte condition can provide guidance in developing high energy density Li-S battery.