A　real　underground　road　tunnel　is　selected　for　analyzing　the　changing　law　of　the　supplementary　air　volume　at　both　ends　of　tunnel　by　a　multi-scale　coupling　method.　The　partial　transverse　ventilation　system　has　been　installed　in　this　tunnel.　For　a　long　tunnel,　one-dimensional　(1D)　model　or　three-dimensional　(3D)　model　has　its　own　limitation　in　the　numerical　simulation.　A　multi-scale　model　which　couples　1D　and　3D　models　has　been　adopted.　The　computed　results　by　multi-scale　couple　model　are　compared　with　those　by　3D　model　in　which　open　boundary　condition　are　set　at　two　portals.　The　results　indicated　that　the　air　supplement　volume　at　two　portals　of　the　tunnel　by　multi-scale　coupling　method　is　less　than　the　recommended　value　of　smoke　exhaust　by　engineering　standard.　In　addition,　the　multi-scale　coupling　simulation　has　obvious　advantages　in　evaluating　the　functional　of　partial　transverse　ventilation　systems.　A　formula　has　been　put　forward　for　the　design　of　smoke　exhaust　volume.　Compared　with　the　recommended　value　of　the　smoke　production　in　the　engineering　specification,　this　smoke　calculation　formula　is　more　operable　and　more　convenient　to　estimate　the　capacity　of　the　exhaust　system　in　design.　©　2017　The　Author(s).