Abstract: The principle of stitching interferometry for testing the large plane optics is presented in this paper. The relative placements of both interferometer and the mirror under test can be adjusted, and interferometric test of subaperture can be done over the whole part of the mirror surface under test. The wavefront data in the overlapping areas of adjacent subapertures are analyzed by using the leastsquare algorithm. The relationship between the tested subapertures is clarified step by step. Finally subapertures are stitched together, and full aperture surface shape under test is synthesized. The error between the spliced surface by stitching interferometry and the actual surface is analyzed, and the accuracy of stitching interferomertry is evaluated. The experiment based on the theory of stitching interferometry was done. The results indicate that the stitching interferometry is an effective means to measure largeaperture optics accurately.