The invention provides a method and a device for processing image, which are used for finding out a common pattern of a plurality of images, namely three or more images. The method comprises the steps: extracting image characteristics of N images; classifying the N images into C layers according to the result of the characteristic extraction to make images having the common pattern gather on a certain layer of the C layers substantially, wherein the C is a natural number and is larger than 2; calculating the average similarity of the N images on each layer; and determining a synthetic image of the layer having the maximum average similarity as an image containing the common pattern, wherein the synthetic image is obtained by synthesizing the N images of the layer on the basis of a reference image of the layer and the reference image is a matched optimal image of one of the N images of the layer and the rest N-1 images. The invention also provides a watermark detection system having the device for image processing. The invention can be used for detecting a watermark from a plurality of document images.