Abstract: On the basis of summing up conventional testing methods for convex mirror, a novel method for testing convex surfaces by subaperture stitching interferometry (SSI) was proposed. A sphere mirror is used as the reference surface, the phase distribution of each subaperture can be measured by the digital wavefront interferometer, and the full aperture surface map can be calculated by stitching several subapertures. The basic principle and theory of SSI were researched, and the stitching processing and prototype for testing convex spheres were devised and developed. A convex SiC sphere with the aperture of 300 mm was tested by SSI with 9 subapertures. For the validation, the sphere was also tested by large interferometer and beam expander, and the peakvalley(PV) and root-mean-square(RMS) residual errors between them were 0.102 and 0.009 , respectively(=632.8 nm).