1,1,2,2-Tetrafluoroethyl 2,2,2-Trifluoroethyl Ether (TFTFE) as electrolyte co-solvent was applied in lithium/sulfur (Li/S) battery for the first time. The ether with partially fluorinated structure reduces the conductivity of the electrolyte but increases distinctly the reversible capacity and coulombic efficiency of the cell, meanwhile decreases the self-discharging loss at the rest period of the cell. The limited dissolution of polysulfides into electrolyte due to the presence of TFTFE is an important factor for the improvement of overall cell performance. Moreover, TFTFE is found to modify the surface film on Li anode greatly, blocking the access of polysulfides to metallic Li and side-reactions between them. TFTFE acts as both polysulfides-restraining solvent and film-forming agent in a Li/S cell system, reflecting the particular functions of the fluorinated ether.