The first problem one may face in studying the thought of Hua Yen Shcool is who is the first patriarch. Then，there comes another topic of heated discussion -- who is the author of Fa Chieh Kuan Men or The Door of the Visualization of Dharma Realm，an important essay on the thought of Hua Yen School. According to 常盤大定 (Ch'ang P'an Ta Ting) and other researchers, the first patriarch of Hua Yen School is Tu Shiun (杜順). Besides, Tu Shiun is considered by most scholars that he is the author of Fa Chieh Kuan Men.
This paper tries not to clarify who is the author of Fa Chieh Kuan Men but to describe the development of the thought of Hua Yen School by means of Fa Chieh Kuan Men. Perhaps, through the clarification of the development of the thought of Hua Yen School，we may understand the content and the author of Fa Chieh Kuan Men.
This paper finds that the thought of Fa Chieh Kuan Men already existed in the time of Fa Tsang. In his essays, Fa Tsang (法藏) devoted a lot to the treatment of the problem of the li or noumenon and shih or phenomenon. Judged from the three visualizations of Fa Chieh Kuan Men，the justification of the no-obstruction between the noumenon and the phenomena is the key to the discussion of the no-obstruction between phenomenon and phenomenon in the All-pervading and All-inclusive visualization.Therefore，Fa Tsang devoted a lot of space to describe it. Accordingly，when in later time，Hui Yuan (慧苑) interpreted the Avatamsaka Sutra with the inter-penetration of Fa Hsion or Phenomena and Nature，he ignored more or less the difference between the noumenon-phenomena and the phenomena-phenomena as well as the boundary of Yuan Chiao or the Complete Teaching and Tsung Chiao or Final Teaching. This is also the reason why Ch'en Kuan (澄觀) attached on Hui Yuan. As to Tsung Mi (宗密)，in General he continued to apply the viewpoint of Ch'en Kuan to interpret Fa Chieh Kuan Men. However，He already regarded the dharma realm of the non-obstruction between phenomenon and phenomenon as the True mind and Original Nature.
