In this paper, a refraction and diffraction mixed compensation test optical path based on computer-generated hologram (CGH) has been proposed, and the combination of the CGH and aplanatic lens is realized to measure the off-axis segment in null test with focal reducing. The total length has been reduced to 1/4 to 1/8 of that of the conventional method. And the adjustment sensitivity of the small focus ratio segment has been improved. The distortion ratio of the interferogram of the large off-axis segment has been optimized from 12.5 to 1.25. The segments with different off-axis distances can be measured rapidly with high precision by replacing different computer-generated holograms and keeping every elements in place. The method is used to complete the processing and testing of a Φ330 mm off-axis trial mirror, and the root mean square (RMS) value of the test surface figure is 0.0290λ (λ is wavelength). The RMS value of the test surface figure is reduced to 0.0267λ, by calibrating the aplanatic lens using a standard spherical mirror.