Ground　penetrating　radar　(GPR)　has　been　widely　used　in　tunnel　karst　detection　as　a　main　tunnel　advance　geological　forecast　mean.　But　its　time　profile　obtained　by　the　analysis　software　in　GPR　can　only　analyze　karst　qualitatively,　and　is　difficult　to　determine　the　size,　location,　shape　of　karst　cave.　In　this　context,　a　single　channel　signal　method　is　developed　for　calculating　karst　position　and　the　size　based　on　GPR　data.First,　analysis　software　in　GPR　are　used　to　determine　which　signal　channel　a　karst　is　located　in,　then　the　data　of　signal　with　regard　to　the　karst　is　extracted　by　self-compiled　program　and　is　transformed　using　the　Hilbert　function　so　as　to　draw　out　the　figure　of　instantaneous　amplitude,　instantaneous　phase,　instantaneous　frequency　of　signal　(denoted　as　three　instantaneous　figure).　and　then　mutation　point　of　＂three　instantaneous　figure＂　of　each　signal　is　integrated　to　determine　the　starting　point　and　the　end　point　of　a　karst.　At　last,　the　caculated　results　of　all　channel　signals　are　united　to　determine　the　karst　position,　size　and　shape.The　proposed　method　is　applied　to　judge　the　position,　size　and　shape　of　the　karst　at　the　section　of　DK189　+　135,　DK189　+　75　in　Zhongfu　tunnel　of　Gan-Long　railway,　which　proved　that　the　method　is　applicable.　©　2016,　Science　Press.　All　right　reserved.