Experiments　were　conducted　with　a　1:20　scale　model　of　tunnel　to　analyze　the　effect　of　blockage　ratio　on　critical　wind　velocity.　The　critical　velocities　under　the　conditions　of　nine　blockage　ratios　at　the　fire　power　of　5.88　kW　were　tested　and　studied.　The　results　showed　that　the　critical　wind　velocity　decreased　obviously　when　the　tunnel　was　blocked　by　vehicle　and　it　decreased　with　the　increase　of　blockage　ratio,　and　the　decreased　proportion　was　slightly　less　than　that　of　the　blockage　ratio.　Data　fitting　was　carried　through　on　the　measured　values　of　the　critical　velocities　to　obtain　the　modified　coefficient　0.548　of　blockage　ratio.　Accordingly,　the　formula　was　proposed　for　calculating　the　critical　wind　velocity　considering　blockage　ratio.　The　relationship　between　the　decreased　proportion　of　the　critical　wind　velocity　and　blockage　ratio　was　contrasted　and　analyzed　based　on　the　results　of　the　tests　and　previous　studies.　It　can　be　concluded　that　the　main　factors　for　the　differences　in　the　relationships　between　the　decreased　proportion　of　critical　wind　velocity　and　the　blockage　ratio　are　the　height　of　fire　burning　surface,　the　location　of　fire　source　(the　relative　position　of　the　fire　source　to　blockage　vehicle　and　whether　the　fire　source　is　in　or　out　of　the　blockage　vehicle)　and　the　open　area　of　vehicle　lateral　wall.　©　2015,　Chinese　Academy　of　Railway　Sciences.　All　right　reserved.