In offset printing system, presensitized plate (PS plate) is one of the key factors that influences the quality of printing products. In this paper, we suggest an application of image processing techniques to improve the accuracy of quality detection of PS plate. We use median filtering to get rid of noises and grayscale stretch to enhance image. An improved adaptive threshold selection method for defect segmentation is proposed based on the characteristic of grads-histogram of PS plate image, and a fast scanning algorithm is put forward to extract features of defects for classification and recognition. The experimental results show that the above methods can obtain the accuracy of 96.7% for defect detection of PS plate and the minimum detectable diameter of defects is 0.2mm, which has reached the enterprise standard for quality detection.