Recurrent aphthous ulcer(RAU)，a common oral mucosal disease in clinical practice，is a disease of aphtha in traditional Chinese medicine(TCM).Because of the complex etiology，unclear pathogenesis，and periodic and repetitive manifestations，this disease is refractory，which seriously affects the patients' quality of life.By summarizing the experience of predecessors and his clinical practice experience，Professor Li Diangui believes that the location of disease mainly lies in the spleen and stomach and is associated with zang-fu organs.The main pathological factor of this disease is turbid toxin，which is caused by the invasion of external turbid toxin or the long-term accumulation of internal turbid toxin.Such toxin causes inflammation and further the onset of RAU.Therefore，the clinical treatment should focus on turbid toxin and adopt the method of removing turbid toxin.The specific treatment schemes should be selected based on the differentiation of five syndromes：burning of heart fire caused by the accumulation of turbid toxin in the heart，burning of stomach fire caused by the accumulation of turbid toxin in the spleen，up-flaming of liver fire caused by the accumulation of turbid toxin in the liver，stagnation of hire and heat caused by the accumulation of turbid toxin in the lung，and consumption of yin caused by the accumulation of turbid toxin in the kidney.Such treatment principle has achieved definite clinical effects and is worth popularizing.