In order to realize sub-aperture stitching measurement of a large-aperture optics, a work-piece localization method based on stereo vision is introduced,and a measurement system is set up. Firstly, the measuring principle of a sub-aperture stitching test is given, and its requirement for the work-piece localization is analyzed. Then, the stereo vision measurement system is introduced and its measuring model is established. After that, the marked points’ 3D coordinates which are rigidly connected with the optics under test are obtained and the transformation matrix is solved out by the quaternion method. Then, the full-aperture phase map is synthesized by mathematically minimizing the mismatch among the overlapping areas of all sub-apertures. Finally, a 150 mm flat and a 100 mm convex sphere are tested using this method. Experimental results indicate that the translation precision and rotation precision of the system are better than 0.1 mm and 0.01°, respectively,which can provide an effective initial value for the stitching algorithm.Furthermore,the measurement error of the system does not accumulate with the motion in its view field,and can satisfy the system’s precision requirements.