Facing　the　elevated　surface　summer　O3　over　North　China　in　recent　years　with　the　continuous　NOx　reduction,　we　conducted　the　ozone-precursor　sensitivity　study　in　summer　(July)　in　Beijing-Tianjin-Hebei　region　(BTH)　under　the　2018　and　2014　emissions,　based　on　WRF-Chem　model.　On　2018　emission　condition,　30%　precursor　reduction　simulations　presented　the　positive　contribution　of　VOCs　and　the　negative　contribution　of　NOx　to　daytime　O3.　The　occurrence　probabilities　of　VOCs-sensitive,　NOx-titration,　mixed　sensitive,　NOx-sensitive,　and　non-sensitive　regimes　respectively　reached　3-49%,　2-82%,　0-7%,　0-6%　and　14-82%　in　the　urban　grids,　and　2-32%,1-19%,　1-6%,　0-5%　and　54-86%　in　the　rural　grids.　For　several　widely　used　photochemical　indicators,　their　values　in　VOCs-sensitive　regime　were　well　separated　from　those　in　NOx-sensitive　regime,　but　the　NOx-sensitive　values　were　very　similar　to　the　nonsensitive　values,　which　implied　the　inefficiency　of　these　indicators　in　indicating　NOx-sensitive　regime.　Finally,　VOCs-sensitive　regime　was　discerned　based　on　the　indicator　HCHO/NO2,　occupying　about　a　third　of　areas　in　morning　and　dusk　but　shrinking　to　about　a　tenth　of　areas　in　11:00-16:00　in　BTH.　And　these　areas　apparently　decreased　from　2014　emission　to　2018　emission.　However,　the　rest　areas　of　this　region　were　under　non-sensitive　regime　but　not　NOx-sensitive　regime,　for　the　noticeable　O3　drop　never　happened　in　NOx　reduction　scenario.　Meanwhile,　the　modeled　O3/PAN　in　the　areas　under　non-sensitive　regime　exceeded　60,　which　also　implied　the　minor　impact　of　local　photochemistry　on　O3　there.　Additionally,　the　responses　of　daytime　O3　to　precursor　emissions　in　the　urban　grids　were　calculated,　declining　by　20.8%　for　NOx　and　6.9%　for　VOCs　from　2014　to　2018.　Therefore,　to　solve　the　ozone　pollution　problem　of　BTH,　the　cross-region　strategy　coupled　with　the　VOCs　and　NOx　co-control　will　be　essential.