According to the study of modern Acupuncture the effect of acupuncture to human body is bidirectional. This paper reviewed TCM classics, and combined with biological theory, tried to explain the mechanism of bidirectional regulation of acupuncture by modern scientific theory. Based on previous study, the author believed that acupoints and internal organ innervated by the same nerve segment formed a relatively close attached structure function unit under the control of sympathetic nerve. Acupoincts innervated by different nerve segment surrounded by this kind of structure function unit formed functional set which is probably reverse adjusted by vagus nerve. Homotopic acupoint and heterotopic acupoints together constitute homeostasis system which could regulate somatic afferent information and balance visceral function. The foundation of bidirectional regulation of acupuncture is adjustment of acupuncture for homeostasis.