The invention discloses a rectangular seal positioning method and a device aiming at a certificate image. The method comprises the following steps of: S1, sequentially judging whether a color of each pixel in an original image is the color of a specified seal, and generating a binary image of original image; S2, generating an edge binary image based on the binary image; S3, setting a window, judging whether a total amount of a foreground ponit in the window in the edge binary image is less than a preset noise threshold, if so, moving the window to rejudge in the edge binary image, if not, calculating a center of gravity of the foreground ponit, and determining whether the centre of gravity is within a preset area, if so, determining that positioning is carried out successfully, and recording the edge binary image in the window, if not, moving the window in the edge binary image and rejudgeing. According to the method and device, some characteristics of a seal image are considered, a foreground image with a specified color is extracted, and the window is set to slid to intercept a rectangle to initially judge a position of the rectangle in the whole image, so that rectangle detection aiming at the seal image in a later period can be carried out within a small scope, and probability of false detection and a calculated amount are reduced.