A carriage module for scanner comprises a case, an image sensor, a mirror set and a lens set. The image sensor, the mirror set and the lens set are arranged in the case and used to guide a reflected light from a document to be scanned as well as convert the reflected light to an electrical counterpart. The case composed of a first shell and a second shell with a larger length than that of the first shell. The second shell is movably arranged in the first shell and can be moved along a horizontal direction.