The　structure　and　characteristics　of　the　two-phase　closed　thermosyphon　with　working　trap　are　described.　A　mathematical　model　is　presented　to　calculate　the　unsteady-state　start-up　process　of　a　room　temperature　heat　pipe.　The　model　is　based　on　the　special　structure　and　operation　conditions　of　the　heat　pipe,　which　is　simplified　to　that　the　steam　temperature　in　the　tube　changes　with　time　only.　The　relation　of　steam　temperature　and　time　is　obtained　by　numerical　calculation　and　used　as　the　temperature　boundary　condition　of　the　measurement　trap.　The　temperature,　velocity　and　pressure　distribution　in　the　working　trap　are　then　solved　by　FLUENT.　The　main　effect　of　ambient　air　on　the　thermometer　well　occurs　at　the　orifice,　while　the　temperature　is　uniform　at　other　locations.　The　calculation　is　in　good　agreement　with　experimental　data,　and　the　maximum　deviation　is　3.7　K　when　the　start-up　time　is　100　s.　The　simulation　and　experiments　indicate　that　water　fluid　filling　volume　has　little　effect　on　the　start-up　performance　of　the　heat　pipe.　The　mathematical　model　and　the　research　methods　provide　a　new　approach　for　the　study　of　special　heat　pipes.　©　All　Rights　Reserved.