Abstract: In order to test large aperture and high steep aspheric surfaces accurately during the grinding and polishing process, a new method combining the subaperture stitching and null compensation was proposed. The principle of the technique was introduced, and a reasonable splicing mathematical model was established. A steep, high order and off-axis aspheric mirror with a aperture of 400 mm×300 mm was tested with the method. The translation and misalignment errors were eliminated by the least-squares fitting and the phase data of the full aperture were obtained, and the surface map after stitching was consistent to the integrated surface map derived by null corrector. Moreover, the errors and accuracy of the experiment were analyzed.