Mapping distortion can be introduced in null testing for off-axis aspheric surfaces, from which the testing results with coordinate deviation cannot be used to guide deterministic optical manufacturing. We propose a correction method for mapping distortion in this study based on the imaging distortion analysis of a null lens and iteration algorithm. We use imaging distortion data to fit the distortion polynomial function by the least-squares method, and we design an iteration algorithm to determine the rotation angle and the position of the null distortion point of the testing results. Finally, the mapping distortion can be accomplished without any markers on the mirror under test. The 280 mm × 180 mm primary mirror and 234 mm × 176 mm tertiary mirror of the hyperspectral camera were tested with a null compensator, and the distortion correction results were obtained using the proposed method. After magnetorheological finishing based on the correction results, the rms of the primary mirror and tertiary mirror reduced from 0.263 λ to 0.013 λ and 0.059 λ to 0.018 λ (λ = 632.8 nm), respectively. © 2022 Optica Publishing Group.