1. A method for image correction of a scanner, wherein the scanner comprises an image sensor and an automatic document feeder, the automatic document feeder has a paperweight mechanism, and the paperweight mechanism is used to press a document flat, the method for image correction of the scanner comprising: marking a first black characteristic region and a second black characteristic region on the paperweight mechanism; using the image sensor to search the first black characteristic region and the second black characteristic region, so as to define a scan line; when the document is to be scanned, moving the image sensor to a position corresponding to the scan line and starting to scan the document, so as to obtain a scanned image; identifying the scanned image to obtain two position information associated with the first and the second black characteristic regions; calculating a tilt angle of the scanned image according to a distance between the first and the second black characteristic regions and the two position information; and correcting the scanned image according to the tilt angle.