1. an image joining method, comprising steps of: (a) obtaining a first image, a second image and a displacement data, wherein the second image comprises at least a line image, and the displacement data comprises a horizontal displacement data and a vertical displacement data; (b) dividing a specific region of the line image as a plurality of image blocks and calculating to obtain a plurality of feature values, wherein each of the feature values is corresponded to one of the image blocks, and the amount of the feature values is equal to the amount of the image blocks; (c) comparing the feature values of the image blocks and utilizing an initial coordinate of the image block corresponding to the maximum of the feature values as a feature point representing the line image; (d) corresponding the feature point to a base point of the first image according to the displacement data and obtaining a first joining point corresponding to the first image and a second joining point corresponding to the second image according to the base point and the feature point; and (e) jointing the first image and the second image through the first joining point and the second joining point, wherein the step (e) further comprises sub-steps of: (e1) calculating to obtain a scaling ratio according to the first joining point, the second joining point and the vertical displacement data; (e2) calculating to add pixels into the first image or the second image according to the scaling ratio; and (e3) overlapping the first joining point and the second joining point and joining the first image and the second image.