The testing method combining systemic wave aberration testing with subaperture stitching interferometry was proposed for measuring large off-axis convex aspheric mirror. The basic principle and process of the technology were analyzed and researched, and the stitching mathematical model was established. When the large convex surface is polished very well and the primary mirror and tertiary mirror of the optical system are fabricated by computer controlled optical surfacing and magneto rheological finishing, the three mirror astigmatism system can be aligned and adjusted. The wave aberration of each field can be tested by aligning and calibrating the optical system successively, and the phase map of the whole aperture can be calculated by the synthetical optimization stitching algorithm and interpolation. With engineering examples, a three mirror astigmatism optical system with a large convex aspheric mirror with the aperture of 292 mm×183 mm was fabricated and tested by the method. At last, the root mean square of the surface error is 0.017λ (λ=632.8 nm).