The concept of "three-yin and three-yang" in Shanghan Lun (Treatise on Cold Pathogenic Diseases), a classic written by Zhang Zhongjing in Han Dynasty, has been always the focus of dispute in successive dynasties. The essence of "three-yin and three-yang" has not been fully revealed up till now. Through studying the six divisions of day and night, the six diseases, the combination of syndromes, the complicated diseases, the complete recovery time and the space division of "three-yin and three-yang", the authors draw a conclusion that the "three-yin and three-yang" in Shanghan Lun is a concept of time-sequence, which is associated with the location of disease in space. So it is suggested that the "six diseases" in Shanghan Lun is a categorization for exogenous febrile diseases, and this categorization reveals a sort of inner relationship between the emergence, development, transformation of the febrile diseases and the time.

Key words: Shanghan Lun, Three-yin and three-yang, Six divisions of day and night, Six diseases