Dayuan Decoction is a classic prescription for treating epidemic diseases.Professor ZHANG Boli，a renowned traditional Chinese medicine(TCM) expert，established the theory of “dampness-turbidity and phlegm-fluid diseases” and clinically treated these conditions using the method of “opening onto pleurodiaphragmatic interspace”.The application of Dayuan Decoction to treat dampness-turbidity diseases has shown clear clinical efficacy.This article first introduces a case study and then explores Professor ZHANG Boli's approach to differentiating and treating dampness-turbidity diseases using Dayuan Decoction.The pathogenesis of dampness-turbidity and phlegm-fluid diseases is primarily due to the internal accumulation of dampness and turbidity in pleurodiaphragmatic interspace，which become difficult to treat and cause prolonged illness.The long-term retention of dampness-turbidity substances leads to toxin formation，which serves as an important foundation for the disease's deterioration.The theory of pleurodiaphragmatic interspace and the essential principles of Dayuan Decoction are outlined as follows:pleurodiaphragmatic interspace treatment emphasizes “opening and expelling” the accumulation，while the approach of “seeking transformation” is key to treatment.The clinical application of Dayuan Decoction follows these principles:the ancient prescription is applied to modern diseases，recognizing the presence of hidden pathogens in the pleurodiaphragmatic interspace.Aromatic drugs are used to expel and resolve the dampness-turbidity accumulation，remove toxins，and treat the disease's deterioration.