This paper summarized professor ZHANG Zhiyuan's experience in using Chaihu (,Radix Bupleuri,). Based on the knowledge he has learnt from the literature of doctors in past dynasties, as well as the experience he has accumulated in the clinical practice, professor ZHANG Zhiyuan makes his own understanding of Chihu in terms of medicinal nature, function and selection. Accordingly, he puts forward the "ascending and descending theory", "heat clearing and cooling theory" and "herbal selection theory". He also gives full play to the application of classical prescriptions including ,Xiaochaihu Decoction, (小柴胡汤) and ,Sini San, (四逆散), and emphasizes the modification of dosage and the combination of medicinals.