In order to correct the distortion introduced in the null testing with computer generated hologram (CGH) for off-axis aspheric, a mapping distortion method is proposed based on affine transformation of the test ray. The distortion mapping function is solved by ray tracing. The high alignment accuracy between the calibration result and the mirror surface is realized with feature points on the mirror, and calibration points are utilized to test the correction error. An off-axis asphere with size of 800 mm×600 mm is tested. After mapping distortion correction, the position error between the test result and the prototype′s profile is reduced from 80 mm to less than 1 mm. The corrected output is applied to polish the mirror by ion beam and its accuracy [represented by root-mean-square (RMS) error] is 0.0162λ (λ=0.6328 μm).