The structure and size range of an acupoint were investigated thoroughly according to the expositions of ancient classics. It was revealed that the ambiguous way of thinking by the ancient Chinese had a great impact on the formation and development of the acupoint-concept. Hence all descriptions of an acupoint’s structure and size range have the characteristics of ambiguity. The structure and size range of an acupoint are determined not only by the outlook of depression and blood vessel areas, but also its relationship with other acupoints in its vicinity. The different manipulations of puncturing recorded in the Yellow Emperor’s Canon of Medicine also show the ambiguous nature of the structure and size range of an acupoint. In theory, an acupoint has been characterized as various forms, which should not be limited to a mere round dot shape.