In order to solve the difficulty of testing large mirror, the sub-aperture stitching interferometry (SSI) is proposed and expatiated. The basic theory and principle of this method are introduced and analyzed. A reasonable stitching algorithm and mathematical model are established based on least-squares fitting, triangulation algorithm, homogeneous coordinate transformation, etc., and the relative program and flow chart are established. Some marked points are used to accomplish the alignments between sub-apertures and calibrate the relationship between the coordinate of the mirror and the pixel. With engineering examples, a large rectangular mirror with an irregular aperture of 720×165 (mm) is tested by SSI. The peak-to-valley and root mean square of the stitched surface error are 0.451 λ and 0.042 λ (λ is 632.8 nm), respectively.