Cheng Xuanying’s (成玄英, ca.600-690) Commentary and Subcommentary on the Preface and Interpretive Principles of the Daodejing (老子道德经开题序诀义疏) was not a passive reproduction or simple exposition of earlier commentarial traditions, but a conscious project of Daoist theological construction and intellectual synthesis.At the macrostructural level, Cheng employed a threefold nested structure of “opening exposition—prefatory principles—subcommentary,” together with a five-level system of textual division and classification, to establish the sacred authority of the Daodejing and provide an orderly exposition of its underlying principles.At the microstructural level, he employed interpretive strategies involving semantic extension, the introduction of polysyllabic terms, and the reconfiguration of conceptual categories, combined with highly dialectical sentence structures such as “questioning and debating /dialectical interrogation” and “double negation,” while adhering to the principle that “the subcommentary should not undermine the primary commentary.” In doing so, he subtly displaced the conceptual core of Wang Bi’s (王弼, 226-249) ontological emphasis on wu (non-being) and constructed an interpretive system integrating Daoist theology with the Chongxuan (“Twofold Mystery”) tradition.Cheng Xuanying’s use of the yishu (“commentary and subcommentary”) genre demonstrates that the transformation of medieval thought was never merely a conceptual game detached from language, but was deeply embedded in historical changes in commentarial structures, textual forms, and modes of semantic interpretation.Reexamining the canonical practice of this intellectual and literary genre not only helps reveal the deeper mechanisms underlying the construction of Daoist doctrine in the Tang Dynasty (618-907), but also provides important grounds for understanding the substance of interactions among Confucianism, Buddhism, and Daoism in medieval China and for developing a Chinese framework for the interpretation of classical texts.