Abstract: Based on optical triangulation measuring principle, an inner diameter non-contact measuring method is presented in this paper. First, the principle of optical single-triangulation which is used for non-contact measurement of inner diameter is analyzed and described. The corresponding formula for calculating the dimension of inner diameter is presented. The mathematics model for solving eccentricity of optical head which is adopted in the optical single-triangulation method is founded. The paper adopts optical single-triangulation measuring principle to design a system of inner diameter non-contact measurement with the technique of laser collimation, modern sensor, servocontrol, computer, etc. In this paper, the constitution and overall structure of the system are described, and also, the accuracy of system is verified by experiments. The measuring system resolving power can reach 0.01 mm, the limited measuring error not more than 0.03 mm. It indicates that the measuring method is applicable.