At present, the domestic measurement of large diameter means still stays on traditional mechanical methods, such as large dry scales, copper tape rulers and π, etc. These methods can only measure parts with IT7 and IT8 precision, for high precision parts. Its size is actually in an unmeasurable state, and the dimensional accuracy of large key parts is an important factor in ensuring the quality of major technical equipment manufacturing, and there are no ready-made instruments sold abroad. Therefore, large-diameter and high-precision measurement is a common measurement technology problem that is urgently needed to be solved in the manufacture of many major technical equipments in large and medium-sized enterprises in China. The aiming and positioning of the large straight-forward measurement points are not well solved at home and abroad. Technical difficulties, this paper proposes a new large-diameter two-point measuring point positioning method and measuring diameter principle, which can be directly applied to large-diameter sites.