T`ien-t’ai School was the excellent school in Chinese Buddhism. The T’ien-t’ai School was established by Chih-i. (A.D. 538-597) He was a great philosopher. He systematized the doctrines of this school. The theory of “Tathāgata inherent evil”（evil immanet in Tathāgata nature）was the special creative theory of Chih-i. This theory also manifested the particular feature of T`ien-t`ai School. This paper attempts to deep inquiry study this thought’s Origin, Method, Content, Principle, Direction and Practice. Finally, I grasped its real meaning, discovered its eternal valuation, and also attained the wisdom of life.