Karst　tiankengs　are　not　only　a　rare　geological　wonder　in　the　world　with　their　huge　negative-landform　in　the　earth＇s　surface,　but　an　ideal　region　to　explore　plant　species　diversity　and　spatial　distribution,　which　attribute　to　an　unique　habitat　formed　by　their　trapped　topography.　This　study　analyzed　the　grassland　plant　community　in　the　＂　Bajiaxiantang　＂　karst　tiankeng,　which　is　a　moderately　degraded　habitat　in　the　Zhanyi　tiankeng　group　in　Yunnan　Province.　The　alpha　diversity　index　was　applied　to　analyze　the　spatial　distribution　characteristics　of　the　grassland　plant　community　species　in　the　bottom　of　this　habitat,　and　the　similarity　of　the　grassland　plant　community　was　compared　to　that　of　the　degraded　doline　near　＂Bajiaxiantang＂　and　the　outside　area　of　the　＂Damaosi＂　primitive　tiankeng　using　a　beta　similarity　coefficient.　The　results　showed　that;　(1)　in　the　bottom　of　the　moderately　degraded　karst　tiankeng,　the　species　richness　and　diversity　of　the　grassland　plant　community　presented　an　＂inverse　depression　effect＂,　where　the　species　richness　and　diversity　were　higher　in　the　center　and　lower　at　the　edge;　(2)　compared　with　soil　resources,　the　growth　and　spatial　distribution　of　plant　communities　in　the　karst　tiankeng　were　possibly　more　sensitive　to　light　resources,　and　the　spatial　heterogeneity　of　light　caused　by　the　height　difference　of　the　tiankeng　pit　wall　resulted　in　a　＂circle　effect＂　of　the　grassland　plant　species　diversity;　(3)　the　isolated　and　protective　function　on　plant　communities　by　the　unique　trapped　topography　of　karst　tiankengs　even　be　reflected　in　degraded　tiankengs,　and　are　more　protective　for　ombrophyte.　With　global　climate　change,　karst　tiankengs　should　be　served　as　important　regional　small-scale　ecological　sanctuaries　to　protect　these　unique　ecosystems,　even　though　they　are　degraded.　©　2018　Science　Press.　All　rights　reserved.