The　reduced　SnO(2()110)　surface　has　been　investigated　by　using　first-principles　method　with　a　slab　model.　By　examining　the　vacancy　formation　energy　of　three　kinds　of　reduced　SnO2(110)　surfaces,　the　most　energetically　favorable　defect　surface　is　confirmed　to　be　the　surface　with　the　coexistence　of　bridging　and　in-plane　oxygen　＇vacancies,　which　is　different　with　the　traditional　odel　by　only　removing　bridging　oxygen.　The　results　of　band　structure　calculations　indicate　that　the　electronic　structure　of　this　defect　surface　is　similar　to　the　SnO　surface.