Defects　are　generally　considered　to　be　effective　and　flexible　in　the　catalytic　reactions　of　lithium-sulfur　batteries.　However,　the　influence　of　the　defect　concentration　on　catalysis　remains　ambiguous.　In　this　work,　molybdenum　sulfide　with　different　sulfur　vacancy　concentrations　is　comprehensively　modulated,　showing　that　the　defect　level　and　the　adsorption-catalytic　performance　result　in　a　volcano　relationship.　Moreover,　density　functional　theory　and　in　situ　experiments　reveal　that　the　optimal　level　of　sulfur　defects　can　effectively　increase　the　binding　energy　between　molybdenum　sulfide　and　lithium　polysulfides　(LiPSs),　lower　the　energy　barrier　of　the　LiPS　conversion　reaction,　and　promote　the　kinetics　of　Li2S　bidirectional　catalytic　reaction.　The　lower　bidirectional　catalytic　performance　incited　by　excessive　or　deficient　sulfur　defects　is　mainly　due　to　the　deformed　geometrical　structures　and　reduced　adsorption　of　key　LiPSs　on　the　catalyst　surface.　This　work　underscores　the　imperative　of　controlling　the　defect　content　and　provides　a　potential　approach　to　the　commercialization　of　lithium-sulfur　batteries.