Tongue　diagnosis　is　widely　used　in　Traditional　Chinese　Medicine　(TCM).　A　tooth-marked　tongue　is　an　important　objective　index　of　the　diagnosis　of　qi　(yang)　deficiency　syndrome.　In　view　of　the　existing　problems　of　the　method　based　on　the　tongue　edge　to　determine　the　tooth-marked,　this　paper　studies　the　improved　method.　Firstly　it　judges　whether　the　top　of　tongue　body　has　recessed　area　and　then　adopts　the　Graham　algorithm　to　construct　the　convex　hull　of　tongue　to　get　accurate　locating　of　tooth-marked.　Secondly　it　chooses　the　optimal　threshold　of　tooth-marked　area　pixel.　The　experiment　results　demonstrate　the　efficiency　of　the　improved　method.