Abstract: For image edge degradation problems caused by straight edge diffraction, an edge degradation model was established. Starting from Fresnel diffraction theory, reasons for image edge degradation were discussed based on mathematical description of image edge degradation of signal and system theory. By analyzing the characteristic of intensity distribution curve of straight edge diffraction in case of different wavelengths of light source and distances from image and object, a simple function was used to represent distribution of straight edge diffraction, and the transfer function of the degradation system was obtained. Then the transfer function of the degradation system was used to realize the restoration of the degraded edge. Experimental results indicate that the presented method has better effects on restoration of the degraded edge caused by straight edge diffraction. The measurement error based on corrected data is 0.02 mm, which reduces 0.04 mm comparing to the measurement error based on original data.