Solvate electrolytes have been proposed to suppress the polysulfide dissolution and enhance the capacity retention of lithium-sulfur (Li-S) batteries. In this study, we report on our use of in situ spectroscopy including Raman and X-ray spectroscopy (X-ray diffraction and X-ray absorption spectroscopy) to investigate sulfur reaction mechanism and the interaction between polysulfide and electrolyte. Raman spectroscopy and cyclic voltammetry obtained from sulfur-carbon cathodes in the conventional ether-based electrolyte show that long chain polysulfides (S82-) are formed in the first reduction process at ~2.4 V vs Li/Li+ and short chain polysulfides such as S42-, S4-, S3․- and S2O42- are observed with continued discharge at ~2.3 V vs Li/Li+ in the second reduction process. The elemental sulfur changes from orthorhombic phase to monoclinic phase in the conventional ether-based electrolyte during the first cycle. In acetonitrile-based solvate electrolyte, in situ spectroscopy results show that short chain polysulfides are formed at the early stage of discharge process and the formation of soluble long chain polysulfides is suppressed. Elemental sulfur with orthorhombic phase is reformed in acetonitrile-based solvate electrolyte during cycling. These results suggest that solvate electrolyte changes the sulfur reaction mechanism. The effect of solvate electrolyte on Li plating/stripping process was also studied. We next propose different solvate electrolytes with low polysulfide solubility and high stability toward Li metal to enhance the capacity retention of Li-S batteries.