Yuan JEN and Baiju-yi are essential writers in the poetic history of the Tang Dynasty. Although their poetic works have both strong and weak points, Yuan and Bai produce a great impact to the poetic style of the Mid-Tang as well as the Song Dynasty. The two men's poetry, one may well say, carries forward Du-fu's tradition and expands it to a wider vision. Su Dong-pe employs four words to describe Yuan's and Bai' poetic style: Light Yuan, Popular Bai. To the four words above, Xu-yi of the Song Dynasty interprets in his "Yan Jou Poetic Criticism" as following: "I (i.e., Dong-po) think that one should have a rigorous approach to find truth, but a generous attitude to treat people." His words are worth pondering. The cultural spirit of the Song Dynasty differs from the magnificent style of the Great-Tang Dynasty. However, it inherits the reasonable and retrospective style of the Mid-Tang Dynasty. And its features happen to be reflected in Su Dong-po's poetic criticism. The thesis will have an overall investigation on the poetic environment of the Mid-Tang Dynasty, Yuan's and Bai's personalities and behaviors, as well as various poetic styles. Furthermore, by analyzing from Su Dong-po's point of view, it will also try to probe the implications of the so-called "light" and "popular." Since I have little talent and less learning, unavoidably I might have limited or naive opinions. I wish what experts and scholars will be kind enough to give me comments as well as advices.