On the basis of summing up conventional testing methods for convex aspheric surfaces,a novel method for testing convex aspheres by subaperture stitching interferometry (SSI) is proposed. A sphere mirror is used as the reference surface,and the phase distribution of each subaperture can be measured by the digital wavefront interferometer. The full surface map is obtained by stitching five subapertures which have been subtracted the theory wavefront error from phase datum and eliminated the translation errors. The basic principle and theory of SSI are researched,and the stitching software and prototype for testing of convex aspheres are devised and developed. A convex SiC asphere with the aperture of 140 mm is tested by this method. As results,the PV and RMS of the surface error after stitching are 0.274λ and 0.024λ (λ=632.8 nm),respectively. For the validation,the asphere is also tested by null compensation,the synthesized surface map is consistent to the entire surface map from the null test,and the difference of PV and RMS error between them is 0.064λ and 0.002λ,respectively. So it provides another quantitive measurement for testing convex aspheric surfaces.