In the null testing of large-diameter and fast-ratio aspherical mirrors, the incident light rays are sharply turned at a large angle within a short distance, resulting in nonlinear distortion of the test result of the interferometer, and the positional accuracy and error removal efficiency of the computer controlled optical surfacing (CCOS) polishing are seriously affected. In order to correct the image distortion of off-axis aspherical mirror in compensation test, a method for correcting the nonlinear distorted image was proposed. The center position of the distortion was determined by the concentric annular method and the mapping relationship between the tested mirror and the interferogram was established by using ray tracing .In addition,the distortion of a 520 mm×250 mm off-axis paraboloid primary mirror of an optical system was corrected.Results show that the position deviation between the correction result and the surface map of the workpiece is reduced to less than 1 mm, which meets the working requirement of CCOS polishing.