Zhiyi, who is the founder of Tien Tai Sect, is known as the fourth-generation forefather in the history of Tien Tai Sect of Chinese Buddihsm. He carried on the practices of the second forefather of Tien Tai, the practice of "three aspects of knowledge and one mind" and the practice of "three meditations of one mind" and developed the zen style of the third forefather, who proposed to lay equal stress on stoppings and contemplations and "Concentration and lsight".He regarded the Saddharmapundarika sutra translated by Luoshen as the traditional sutra of Tien Tai Sect, Saddharmapundarika sutra belongs to the early Maha-prajna-paramita sutra, but its thought is interlinked with Nirvana sutra, which is the last word of Buddha according to legend. Therefore, Zhiyi can compromise the thought of the Prajna and the Buddhist nature and assimilate the thought of "the Delun religious sect", "the Shelun religious sects " and "the Sanlun religious sect"critically. His thought centered on the Dharmata theory and developed the three dogmas of "three meditations of one mind", which means the three dogmas, namely, void, unreal and mean are co-existent and integrated,to enter into the state of "An idea,Three thousand events"This thesis analyzes his thought from three aspects by four special projects. Firstly, I review the history of the "cause and effect" thought which is the foundation of Buddhism. The thought has experienced the stage of original Buddhism, the stage of Sectarian Buddhism, the stage of Middle way of Mahayana, the stage of Yogachara and so on.Based on the three stage development of the Cause Theory during the history of the Buddihsm in India:"the sin cause","the feeling cause",and " the different categorists cause",to study the different train of thoughr about the Cause Theory of Zhiyi’s, and to reveal the "three meditations of one mind",and the state of "An idea,Three thousand event".Secondly, I explain the connotation of dharma-mimitta, the nature of beings, dharma-dhatu, the nature of idea by the analysis of the theory of dharma of Buddhism. And by the research of the contrast between the nature of beings and the dharma-dhatu and the contrast between the nature of idea and dharma-mimitta to explain the state of the Buddha in the practice as Tien Tai Sect’s theory.From the ultimate realm of the Buddha, the nature of beings,the beings,and the nature of ideas, three forming an organic whole. Thirdly, To reveal how to enter into the state of "An idea,Three thousand event".concretely Fourthly, analyze the theory of marking the religious secof Zhi based on Saddharmapundarika sutra. He concluded his predecessors’s theory to "three groups in South,and seven groups in North." and evaluated them respectively. And then he proposed the most typical "five period and eight teachings "to mark the religious sect。The five periods is a classification of Shakyamuni Buddha’s sutras according to the order in which they were expounded. The eight teachings consists of two sub-classifications:the four teaching of method and the four teaching of doctrine. The four teaching of method, a classification by method of teaching,are the sudden teaching,the gradual teaching, the secret teaching and the indeterminate teaching. The four teachings of doctrine, a classification by content, are the Tripitaka teaching, the connecting teaching, the specific teaching and the perfect teaching. The four teachings of doctrine is the key point of marking the religious sect, so it is also called "Tien tai fou teachings".The Buddhism comes through the last stage of Han, Wei, Jin, and Nan-bei Dynasties in the central mainland and the Tang Dynasty is the heyday of it. Zhiyi’s thought was the first arising typical thought at that time. It indicates that Buddhism, which is different from the local culture of China was really absorbed and had a further development in China for the first time. Zhiyi’s advanced thoughts give many inspiration to the development of Buddhism during the Sui and Tang Dynasties.