A new model developing knife-edge test to a quantified method is built in this paper. Based on knife-edge test and image-processing technology, the local surface deformations on aspheric surface can be characterized. The image-receiver CCD (756x504) can assure that the deformation on the little curved face can be measured and detected sensitively. For reducing the influence of the vibration of optical and image-receiving subsystems and offset of images, the least-squares procedure has been used to process images for three times. Then all of the images will be cut at the least-squares circles and moved to the same position, so every pixel on all images can be matched without any offset. In addition, the authors checked the validity of new method by measuring the characterization of parabolic surface under manufactured in our laboratory, and by processing data with simulation method on computer. The result of experiment and calculation can both approve that the least squares techniques is effective.