To solve the non-uniform background problem induced by the illumination with uneven light intensity in the system of detecting surface imperfection on large-aperture optical components based on the machine vision, an image background correction method is adopted based on the mathematical morphology. An image segmentation algorithm of image gradient in combination with Otsu method is proposed. The experimental results show that the proposed method has the good anti-noise performance and high extracting accuracy for the blurred images at a certain degree.