The　critical　velocity　for　preventing　the　smoke　backflow　in　fire　is　the　key　index　for　design　of　ventilation　and　smoke　extraction　systems,　and　it　is　also　one　of　the　focuses　of　studies　on　tunnel　fire.　Researches　on　the　critical　velocity　have　obtained　some　results　about　the　backflow　of　pure　smoke,　but　the　critical　velocity　of　the　smoke　backflow　with　flame　has　not　been　paid　enough　attention　to　in　such　underground　space　with　large　slenderness　ratio.　Taking　the　Beijing　subway　tunnel　as　an　example,　the　critical　velocity　of　backflow　with　flame　with　the　on-fire　train　remaining　in　the　tunnel　was　analyzed　through　small-scale　model　experiments　and　numerical　simulation.　Considering　the　train-tunnel　dimension　ratio　and　the　role　of　thermal　radiation　impact　on　the　critical　velocity,　a　new　formula　for　calculating　critical　velocity　is　deduced　through　the　energy　equation.　The　formula　is　compared　with　the　existing　formulas　for　calculating　the　critical　velocity　of　the　pure　smoke　backflow,　such　as　Oka-Atkinson　formula,　Wu-Bakar　formula,　etc.,　and　the　results　show　that　the　new　formula　is　more　suitable　than　the　others　for　calculating　the　critical　velocity　of　the　backflow　with　flame.　It　is　also　found　that　the　critical　velocity　is　not　exactly　the　same　for　fires　on　different　parts　of　the　train.　When　fire　occurred　in　the　center　and　the　head　of　the　train,　the　critical　velocity　is　similar;　but　it　is　different　from　fire　occurred　at　the　end　of　the　train.　When　fire　occurred　in　the　train　within　the　tunnel,　the　critical　velocity　is　different　from　the　fire　occurred　in　the　tunnel　without　the　train.