Karst　tiankengs　are　rare　natural　habitats,　having　a　local　microclimate　different　from　surrounding　regions.　A　contrast　study　on　plant　communities　at　the　inside　and　outside　of　the　primitive　tiankeng　was　carried　out　by　performing　the　qualitative　analysis　of　species　compositions　of　arborous　and　shrub　layers.　We　found　that　plant　communities　in　the　primitive　tiankeng　belong　to　the　subtropical　moist　evergreen　broad-leaf　forest,　whereas　those　outside　the　tiankeng　belong　to　subtropical　semi-moist　needle-broadleaved　mixed　forest.　Trapped　habitat　of　primitive　karst　tiankeng　protects　the　plant　communities　significantly　different　from　those　in　external　karst　ecosystems,　so　karst　tiankeng　has　the　great　value　in　the　plant　species　protection.　Although　the　trapped　habitat　decreases　plant　species　abundance　inside　tiankeng　to　some　extent,　highly　diverse　shrub　species　are　present　in　the　inside-tiankeng　plant　communities,　and　the　primitive　karst　tiankeng　plays　an　important　role　in　the　plant　diversity　protection.　The　primitive　karst　tiankeng　is　an　important　refugia　for　plant　not　only　as　a　plant　species　protection　library,　but　a　plant　diversity　protection　library.　When　implementing　measures　for　the　reduction　of　damages　to　biodiversity　due　to　global　climate　changes　and　human　activities,　more　attention　should　be　paid　to　the　primitive　karst　tiankeng　as　a　small　ecological　refugia　and　biodiversity　protection　library.