Based　on　the　multi-　scale　coupling　method　which　field　model　and　network　model　are　combined,　the　smoke　spread　in　the　underground　unidirectional　tunnel　was　studied.　We　compared　with　the　effect　of　smoke　diffusion　during　different　number　of　jet　fans　operated.　Results　show　that　when　three　sets　of　fans　are　operated,　there　is　a　backlayering　of　smoke　which　will　influence　the　simulation　of　the　model,　thus　we　cannot　get　a　steady　state　of　smoke　backlayering.　When　four　sets　of　jet　fans　are　operated,　the　backlayering　of　smoke　is　eliminated,　the　result　of　simulation　shows　more　reasonable.　However,　it　is　difficult　to　maintain　a　steady　stratification　in　the　downstream　of　fire.　The　results　of　the　study　can　provide　a　reference　for　the　design　and　operation　of　jet　fans　in　the　underground　unidirectional　tunnels.　©　2017　The　Authors.　Published　by　Elsevier　Ltd.