Abstract: The Yin-Fire theory is the core content of Li Dongyuan's academic thought and has profound and extensive clinical guidance value. This paper analyzes the connotation and background of the Yin-Fire theory, and proposes that the core pathogenesis of Yin-Fire syndrome is qi deficiency without movement, and stagnation turning into fire; qi deficiency without rise, and dampness and turbidity flowing downward; qi deficiency without astringent, and Xiang-Fire rushing upward; qi deficiency and fire hyperactivity, which hurts body fluid and burns blood. This article summarizes the key points of diagnosis and treatment of various diseases by sorting out the commonly used prescriptions and medicines of Yin-Fire theory and its clinical application in miscellaneous internal medicine diseases, and deepens the understanding of the connotation of Yin-Fire syndrome.