A　critical　velocity　for　preventing　the　smoke　backflow　in　fire　is　the　key　target　in　the　designing　of　ventilation　and　smoke　extraction　systems.　Researches　on　the　critical　velocity　have　obtained　some　results　for　the　backflow　of　the　pure　smoke.　But　the　critical　velocity　of　the　smoke　backflow　with　flame　has　not　been　paid　enough　attentions　to　in　underground　space　with　large　slenderness　ratio.　Taking　Beijing　subway　tunnel　fire　for　example,　while　the　on-fire　train　remaining　in　tunnel,　the　critical　velocity　of　backflow　with　flame　was　analyzed　by　some　small-scale　model　experiments　and　numerical　simulations.　Considering　the　train-tunnel　dimension　ratio　and　the　role　of　thermal　radiation　impacts,　introducing　the　concept　of　the　wall　endothermic　percentage,　a　new　group　of　formulas　is　deduced　to　calculate　the　critical　velocity　of　the　smoke　backflow　with　flame.