A novel quantitative shadow testing equipment is presented based on the digital imageprocessing technique for characterizing local surface deformations on optical surface. Two magnetic stepping motors control the positions of the knife-edge for cutting the wave-front reflected back from the aspheric surface separately. At each position of the knife-edge blocking the image on the focal plane, it is recorded and analyzed by an image-processing set-up behind the knife-edge under computer control. Based on a group of images recorded, the analysis of local slopes is performed by picking up characteristics of illumination levels. Further the relationship between the changes of illumination level and deformations on surface is built on the principle of Fourier Optics. The results of tests approve that the method is effective on determining the characteristics of two parabolic surfaces using magnetorheological finishing.