Chinese medicine (CM) theories attach great importance to the interaction between the human body and the environment. From this entry point, based on a great number of related CM literatures and knowledge of modern medicine, this paper investigated the essence of chilliness and cold limbs of yang deficiency syndrome. The differences on cognition and solution to this problem between CM theory and modern medicine theory were also analyzed. Firstly, the close relationship between the CM concepts of yang deficiency, "external cold", and winter was put forward after a review of yang deficiency in CM scripture Huangdi Neijing. Chilliness and cold limbs of yang deficiency patients was believed to be highly similar to the body state of "external cold" of healthy people in winter. Secondly, the state of "external cold" was described as a reduction of body surface blood flow (heat) with references to the adaptability to cold stimulation. Then according to a collection of physiological and pathological factors which may cause such reduction, we proposed that the essence of chilliness and cold limbs of yang deficiency patients may probably be the state of reduction of body surface blood flow (heat) due to some pathological factors already known as heart failure, poor circulation, and thyroid dysfunction. Thirdly, by using the method of "syndrome differentiation through formula effects assessment", this recognition was confirmed because the pharmacological activities of cardiotonic or peripheral vasodilating, or surface temperature increasing were found in a great number of single medicines and prescriptions applied to yang deficiency syndrome in CM.