Abstract: In order to measure the three-dimensional shape, spherical diameter and sphericity error of the large-size spherical parts, a light-pen vision measurement system is designed. The system is mainly composed of high-resolution camera, light pen and supporting image processing software. Firstly, this paper introduces the measurement principle of the system, and then gives the key technologies in turn: the structural design of the light pen and the light pen calibration method based on close-range photogrammetry, and the method of establishing the conversion relationship between the light-pen coordinate system and the camera coordinate system by using the homography matrix. The accuracy evaluation experiment based on grating ruler shows that the relative measurement accuracy is better than 0.05 mm/m in 2000 mm×1500 mm field of view. Taking the large-size metal ball as the measured object, the three-dimensional coordinates of multiple points on the balll surface are extracted to generate the fitting ball, and then the sphericity error is calculated to be 2.068 mm.