Under the current situation of the rapid development of major equipment and technology, the research on measurement equipment, suitable for requirement of large-size application, is urgency and important. A light pen measuring system based on binocular stereo vision measurement principle is researched. The composition and working principle of the system are analyzed. Subpixel marginal fitting method is adopted to obtain the precise position of the center coordinates of the point of the coded marks. A mathematical model of the camera is Established, and the corresponding relations between the three-dimensional coordinates of the mark points in space and the two-dimensional coordinates of the same points in the image are deduced. In order to improve the speed and accuracy of image matching, the light reflection coded mark stickers are adopted as the marks of the light pen for imaging. The research on the key technologies of the light pen measuring system provides a theoretical basis for the further striking of three-dimensional coordinates of the space point, for the 3D reconstruction of the space model, and for the realization of software technology .