Abstract：The theory of life gate is an important part of the basic theory of Chinese medicine. The discussion of the life gate in Nanjing laid the foundation of the theory of life gate. With the study，supplement，and development of the theory by successive generations of physicians， an independent system about life gate has been gradually formed， which not only greatly enriches the theory of Chinese medicine but also provides an effective guide to clinical practice. CHEN Shiduo， a physician in the late Ming and early Qing Dynasty， discussed life gate in his work Waijing Weiyan. Different from the argument that “the life gate is the eyes” in Huangdi Neijing，he suggested that the life gate in the kidney is the governor of twelve viscera and bowels，and all these internal organs rely on the life gate. He held that “only when there is life gate can viscera and bowels be normal”，and “only with life gate fire can the engendering transformation of meridian and collateral work”. Besides， he emphasized that human growth depends on fire and need to be nourished by water. As for clinical practice， he suggested that we should not only focus on warming and tonifying the life gate， but also pay attention on nourishing yin； because life gate fire is the fire within the water， so， only when yang is tonified by enriching yin can engendering transformation normally and continuously function. By studying the origin and development of the theory of life gate in Waijing Weiyan，this paper further expounds on essence and function of the life gate on the basis of the viewpoints held in the work and discusses the clinical application of this theory by taking the diagnosis and treatment of wasting-thirst as an example.