It　is　important　to　keep　fire　safety　for　subway　system　especially　for　subway　tunnel.　This　paper　selects　a　2.2　km　tunnel　of　Beijing　subway　as　the　research　object,　and　studies　fire　scenario　for　a　stopped　train　with　natural　ventilation.　The　multi-scale　method　where　a　three-dimensional　field　model　and　a　one-dimensional　network　model　are　combined　is　used　to　simulate　smoke　propagation　in　a　tunnel　fire.　The　effects　of　train　length　on　the　smoke　propagation,　temperature　and　visibility　distribution　are　analyzed.　The　results　show　that　during　the　initial　period　of　120s,　the　influence　of　train　length　on　temperature　distribution,　visibility　distribution　and　smoke　distribution　is　not　obvious.　However,　after　120s　period,　smoke　distribution　becomes　more　concentrated　with　the　increase　of　train　length,　and　the　impact　of　stack　effect　is　more　obvious.　For　a　shorter　train,　the　concentration　of　smoke　is　higher.　The　temperature　distribution　inside　the　60　m　train　is　higher　than　those　of　120m　and　160m　trains.