In order to overcome the difficulty of testing the surface shape of large mirror, the subaperture stitching interferometry (SSI) is introduced. The basic principle and flow chart of SSI are analyzed, the synthetical optimization stitching model and effective stitching algorithm are established based on triangulation algorithm, least-squares fitting and homogeneous coordinates transformation etc. With engineering examples, a large sic mirror with the aperture of 800 mm is tested by SSI with 7 subapertures. The coordinate transformation between digital controlled fine processing the mirror surface and pixel coordinate is fulfiled by bench mark and iterative algorithm, so it provide the basis for the subsequent the physical coordinate of the large mirror.