1. A three-dimensional scanner, adapted to build a digital three-dimensional model related to a three-dimensional object, comprising: a light source module, configured to emit a beam; a screen, disposed on a transmission path of the beam, and having a projection surface facing the light source module; a rotary platform, configured to carry the three-dimensional object, and disposed between the light source module and the screen, wherein the rotary platform is configured to rotate the three-dimensional object to a plurality of orientations about a rotating axis, so as to form a plurality of object shadows of the three-dimensional object respectively corresponding to the orientations on the projection surface of the screen; an image capturing unit, configured to capture the object shadows from the projection surface of the screen to obtain a plurality of object contour images; and a processor, coupled to the image capturing unit, and configured to read and process the object contour images to build the digital three-dimensional model related to the three-dimensional object according to the object contour images, wherein the processor compares an initial object contour image corresponding to the three-dimensional object at an initial orientation of the rotary platform with a final object contour image corresponding to the three-dimensional object at a final orientation where the three-dimensional object is finally rotated to obtain a common central axis of the object contour images.