Abstract: In order to solve stitching testing problem of large-diameter optical flat mirror, based on triangulation and least squares fitting, a reasonable stitching algorithms and mathematical models were established and relative program was written. With engineering examples, a rounded rectangle plane mirror of 612 mm180 mm was stitching measured with a 600 mm interferometer. In the testing, the relative position between the sub-apertures was determined based on the target, completing the alignment between the sub-apertures, and based on different mirror locations, the repeatability of stitching testing was verified with several experiments. The experimental results show that there is no trace in the stitching result and the relative deviation of the PV and RMS between the two experiments based on different mirror location are 2.07% and 0.52%. The stitching result is consistent and the reliability and accuracy of the stitching testing are verified.