"Yi Wai Biezhuan (Yu Yan)" is a Taoist book written by Yu Yan, a Taoist Yi scholar in the late Song Dynasty and early Yuan Dynasty, which explains the Taoist Yi Xue (Xiang School) and Alchemy. This book is accompanied by pictures and text. By analyzing the mystery of "Xiantian Diagram" and combining it with Wei Boyang's "Shen Tongqi", it deeply explains the theory and method of inner alchemy cultivation. In his book, Yu Yan closely combined the theory of Yi studies with health-preserving practices, which had a profound impact on the development of Taoist inner alchemy in later generations. This book is not only an important document of Taoist Yi studies, but also a rare and valuable information for the study of Taoist health and inner elixir cultivation.
