Researches　on　the　occurrence　of　backdraft　in　compartment　fires　have　obtained　some　results　in　surface　buildings.　But　the　backdraft　taking　place　in　subway　tunnels,　i.e.,　in　underground　spaces　with　large　slenderness　ratios　has　not　been　paid　enough　attention　to.　Experiments　have　been　conducted　to　investigate　the　difference　between　the　two　kinds　of　backdrafts　by　the　1/8　reduced-scale　subway　tunnel　fire　model.　The　results　indicate　as　follows:　the　key　parameter　determining　the　occurrence　of　backdraft　in　subway　tunnel　fire　is　the　mass　concentration　of　the　volatile　component　of　the　fuel　and　it　has　a　critical　value;　the　mass　concentration　of　the　volatile　component　changes　with　different　ventilation　conditions,　its　critical　value　is　8.78%　in　the　single-tunnel　under　natural　ventilation　and　11.71%in　the　double-tunnel　under　mechanical　ventilation;　the　backdraft　in　the　single-tunnel　under　natural　ventilation　is　similar　to　that　in　the　compartment,　and　the　ignition　delay　is　the　gravity　flow　delay;　the　backdraft　in　the　double-tunnel　under　mechanical　ventilation　is　different　with　that　in　the　compartment,　and　the　ignition　delay　is　decided　mainly　by　the　ignition　source　delay;　the　backdraft　is　influenced　by　the　humidity　of　the　fresh　wind　flowing　to　the　fire　source,　the　humidity　can　be　increased　to　inhibit　the　occurrence　of　backdraft　and　an　optimized　match　exists　between　the　humidity　and　the　velocity　of　the　fresh　wind;　it　is　economic　and　feasible　to　inhibit　backdraft　by　humidifying　the　fresh　wind　flowing　into　the　tunnel　and　the　method　has　important　reference　to　subway　fire　fighting.