Abstract: In order to obtain the figure error of the whole large convex aspherical mirror, a new method for testing large convex asphere by subaperture stitching and interferometry so called SSI was proposed. A perfect sphere mirror was used as the reference surface, the phase distribution of each subaperture could be measured by the digital wavefront interferometer, and the full surface map could be calculated by stitching several subapertures. The basic principle and theory of SSI were studied, the stitching process and prototype for test of large aspheres were devised and developed, and the setup of SSI for measuring of large aspherical surface was designed and manufactured. A convex SiC asphere mirror with the aperture of 260 mm was tested by SSI with 9 subapertures. For the validation, the aspheric mirror was also measured by null compensation, the difference of PV and RMS error was 0.043 and 0.021, respectively ( is 632.8 nm). It provides an another accurate testing method for aspherical surface especially for large convex aspherical mirror.