This paper introduces Professor Liu Qiquan’s clinical experience in treating recurrent aphthous ulcers by syndrome differentiation and corresponding treatment. It is considered that the accumulation of turbid toxin is the key pathogenesis. The lingering of turbid toxin damages healthy qi and causes aphthous ulcers to recur. Once the disease becomes chronic it affects collaterals, stagnating qi and blood flow, causing long-term fester and difficult-to-heal wound. During the treatment, it is essential to resolve turbidity and remove toxin. Meanwhile, take the morphology and location of aphthous ulcers into consideration and treat according to syndrome differentiation. As long as we adopt multiple measures simultaneously, treat both internally and externally together with daily care, and combine treatment and adjustment, satisfactory curative effect can be achieved. One cured case is exemplified.