The　flame　ceiling　jet　is　easily　formed　while　the　on-fire　train　stops　in　the　tunnel.　The　iterative　formula　is　derived　to　calculate　the　average　temperature　of　the　flame　ceiling　jet　when　the　fire　source　is　set　on　the　center　of　the　train　by　establishing　a　one-dimensional　model　of　the　unit　control　volume,　and　the　heat　exchange　is　considered　among　the　flame　smoke　and　the　wall　of　the　tunnel　and　the　train.　Undetermined　coefficients　in　the　iterative　formula　are　determined　by　small-scale　model　experiments　with　the　scale　factor　of　1:　8　and　numerical　simulation.　The　calculated　values　of　temperature　by　the　iterative　formula　are　consistent　well　with　the　measured　values,　and　the　maximum　error　of　calculation　is　no　more　than　7%,　which　indicates　that　the　iterative　formula　is　reliable.　The　results　show　that　the　distributions　of　the　average　temperature　of　flame　ceiling　jet　are　in　the　exponential　form,　but　there　are　some　differences　between　that　above　the　train　and　that　in　the　tunnel.　Two　prediction　formulas　for　the　flame　ceiling　jet　average　temperature　changing　along　the　longitudinal　tunnel　are　obtained　by　the　sub-fitting　method.　The　results　can　provide　reference　for　the　alarm　parameter　selection,　device　settings　in　tunnel,　safety　evacuation　of　the　passenger,　as　well　as　the　analysis　of　the　destructive　effects　on　the　tunnel　lining　structure　caused　by　tunnel　fire.