Abstract: In order to solve stitching testing of precise optical flat mirror, based on least squares fitting, we established reasonable stitching algorithms and mathematical models. In addition, relative program was written and a 120 mm precise flat mirror was tested with stitching method. A marked point was used to accomplish the alignments between subspertures. At the same time, relative stitching accuracy was analyzed based on the full aperture testing result and self-examination subaperture testing result. It is proved from the experiment results that there is no gap in the stitching result. The peak value(PV) and root-mean-square(RMS) errors between stitching results and full aperture testing results are 0.020 and 0.002 respectively. The stitching result is in consistent with the full aperture testing result and self-examination testing result, respectively, which proves the reliability and accuracy of the stitching.