Decoctions have been always playing an important role in Traditional Chinese Medicine (TCM). In this paper, we presented a discovery method of the knowledge of traditional classical prescription (TCP) in TCM. This work was based on the theory of formal concept analysis (FCA), and by the construction and study of an attribute partial order graph of the context of Cassia twig decoction and its derivatives that was chosen from Zhongjing Zhang’s “Treatise on Febrile Diseases”，a historical masterpiece in TCM. The visualized results indicated that we can obtain complete hierarchical structure and find knowledge from the partial-order attribute diagram constructed from the context of TCPs.