Imaging synthesis plays an important role in the defects testing of large aperture optics. To make sure the successful synthesis of sub-apertures, the template matching is used for sub-aperture stitching, by analyzing all kinds of stitching method and combining the needs of defects testing of large aperture optics. Template matching gets great success in the experiment of defects testing of large aperture optic, so it is useful for detecting surface defects of large aperture optics accurately, efficiently and automatically.