The style of advocating fantasy (尚奇) in Mid-Tang poems was a phenomenon of creation integrated by multi-elements of expression. The variation in the way of imagination about immortals in some ancient-style poems from Tianbao (天宝) period to Zhenyuan (贞元) period, and then to Dali (大历) period can be seen as one of the origins of this style. Its way of imagination was fundamentally characterized with the construction of the immortal world according to the logic in real life. This variation of ideas was partly caused by the prosody of ancient-style poems, and was also based on the social mentality about imagined immortals in the Mid-Tang Dynasty. After the armed rebellion led by An Lushan and Shi Siming (安史之乱), the emperors advocated Taoism for the sake of seeking for a way to eternal life instead of seeking advice about how to govern the country. The popularity of Taoism and Taoist temples made the immortal world visible, strengthened people’s knowledge of the immortals, and even led some literati in believing that to be converted to Taoism would be the final end of their lives. Thus, the secularization of the imagination about the immortals in Mid-Tang poems became the inevitable tendency. Although the immortal world blended with reality was short of the rationality and profundity with which the literati of the early Tang Dynasty thought about the Way of Nature (元化之道), it opened up a new horizon of romantic imagination for the poetic history of the Tang Dynasty.