"Chou I Tshan Thung Chhi" has been an enigma of the science of physiological alchemy for a long time. Some people think that it is the combining of exoteric alchemy (wai tan) and esoteric alchemy (nei tan). Some think that it is only the exoteric alchemy or the science of cultivating the spirit by nature. These three sects have their own viewpoints. Therefore, the mystery of physiological alchemy becomes even harder to understand. It is more confusing and absurd when involved with fairy tales. The procedures of the method are often inexplicable. Students of it often get lost. Chapters 27, 34, 70, 71 and 89 in "Chou I Tshan Thung Chhi" clearly demonstrate that sexual activities, nourishing the chhi, gymnastics, worship, meditation without sleep and dietary have departed form Tao. The principles of cultivating the spirit should be “nourishing the mind by interior nature of Huang-Di Lao Tzu; … always embracing oneness (Tao), … disregarding dietary eight minerals.” In this way, I assume that physiological alchemy in this book means breathing exercises to cultivate the spirit without dietary. It inherits the philosophy of concentrating the chhi and being tender of Lao Tzu, aided by the thinking of Chinese medical science; and the abstract principles of using the changes of kua phase (divinatory symbols) to interpret the method. All these words are only borrowed symbols. It may not match with the original meaning of the words. Here I try to interpret the procedures of the method from such viewpoints of cultivate the spirit by nature. It is divided into 4 themes: (1) exercises to cultivate the spirit. (2) The relationship between the advancing phase 9chi Yang huo) & the retiring phase (thui Yin huo) with the heating-time. (3) The possible principle of the interaction between vital mercury and vital lead (Long-hu-jiao-gou). (4) Reinterpretations of Da-zhou-tian & Ziao-zhou-tian.