1. An image processing unit with one button trigger, comprising: a button for inputting a trigger signal, so as to start on the operation of the unit; a prescan module for prescanning a target at a first resolution, which is lower and proportional to a second resolution that is assigned by a user, after the operation of the unit is started, thereby obtaining a first image area; a target image calculation module for calculating the first image area; a target image marking module for marking out the target image contour in the first image; a scan module for scanning the target at the second resolution assigned by a user, thereby obtaining a second image area in proportion to the first image area and finding out the target image contour in the second image area according to the proportional relation between two image areas; a target image processing module for filtering impurities and straightening on the target image of the second image area; and a crop module for abstracting the target image by the marked contour in the second image and trimming the edge of the abstracted target image to get a target the same as size as the scanned target.