A　cross　type　interchange　subway　station　was　selected　as　research　object.　Fire　happened　in　the　3rd　underground　subway　platform　while　the　metro　train　in　the　neighbor　line　passed　the　2nd　underground　subway　platform.　Piston　effect　by　passing　train　would　be　produced.　It　would　influence　the　smoke　propagation　in　the　bottom　platform.　Fire　dynamics　simulator(FDS)　software　was　used　to　simulate　the　piston　effect　by　comparing　two　fire　conditions　where　trains　keep　passing　by　or　not　on　the　neighbor　platform.　Smoke　concentration,　temperature　and　visibility　on　3rd　underground　platform　were　calculated.　Results　show　that　piston　wind　effect　would　enhance　destabilization　on　smoke　layer　and　do　harm　to　passenger　evacuation.