Understanding　the　quantum　effect　in　the　cross-conjugated　system　is　of　fundamental　significance　in　molecular　electronics.　In　this　study,　four　molecules　Xa-O,　Xa,　BP　and　BP-O　were　synthesized　to　investigate　the　destructive　quantum　interference(DQI)　of　a　carbonyl　bridge.　The　single-molecule　conductance　measured　by　the　scanning　tunneling　microscope　break　junction(STM-BJ)　technique　demonstrates　an　increase　in　the　conductance　from　molecule　BP-O　to　molecule　Xa-O　as　the　cross-conjugated　system　is　extended.　Theoretical　calculations　show　that　the　explicit　DQI　feature　is　presented　in　BP-O　but　absent　in　Xa-O,　which　indicates　the　removal　of　DQI　in　the　restrained　structures　and　results　in　the　conductance　enhancement　in　Xa-O.