Being of higher rank in function than pectoral qi or middle qi, great qi is the fundamental qi of all, and the key to balance yin and yang within human body and to recovery from illness. There were slightly different understandings about great qi among doctors of various dynasties. Influenced by traditional Confucius way of thinking and evolved with the observation of relations between nature, human, and illness, the theory of great qi sinking was proposed by Zhang Xichun, which had been an underlying basis for doctors afterwards. Through research on source, meaning, and the sinking theory, the author pointed out three shades of implications, that is, the sinking of great qi may be caused by deficiency, or only the lack of raising strength, or stagnation of phlegm, blood stasis, or pathogen, etc.