Sun Deng of the Eastern Jin Dynasty is the first man who annotated Laozi with concept of Twofold Mystery. From the Southern and Northern Dynasties to the Sui and Tang Dynasties, lots of Taoists commentated Laozi and Zhuangzi or elucidated Taoism with Twofold Mystery. Posterity called them School of Twofold Mystery. Cheng Xuan-ying is important representative of School of Twofold Mystery in Early Tang Dynasty. Dao De Jing Yi Shu is the foundational work of Cheng Xuan-ying's thoughts. The content is to discuss Metaphysics with Twofold Mystery and narrate the process of cultivation and realization of pure mind based on mind and nature. This book's academic status can be generalized into three aspects: Firstly, as far as Laoism is concerned, it has color of linking academic and religion and concluding all theories of predecessors. Secondly, as far as development of History of Chinese Thoughts is concerned, it seems to be seen as the third period of Taoism thoughts' development after Xuanxue of the Wei and Jin Dynasties. It is a fusion of Xuanxue (literally "mysterious learning", also known as Neo-Taoism) and Buddhism Prajñā (Wisdom in Buddhism). Thirdly, as far as influence is concerned, it is the best school that is rich in philosophy significance in Taoism history. The thoughts influenced Science of Mind-Nature and Inner Alchemy (Neidan) of the Song and Yuan Dynasties. Moreover, Twofold Mystery and Theory of Mind-Nature were likely to enlighten the development of Neo-Confucianism.