A height adjustment structure for an image-scanning device. The scanning device has a guiding rod, an optical system and a document panel. The guiding rod and the document panel are fixed relative to each other. The optical system includes a box body and an axial rod bearer that can slide along into the guiding rod. The height adjustment structure includes a first positioning section and a second positioning section. The first positioning section is attached to one side of the box body and the second positioning section is attached to one side the axial rod bearer that faces the first positioning section. The first positioning section and the second positioning section has hooking mechanism, screwing mechanism or teeth-meshing mechanism for fixing the box body relative to the axial rod bearer and hence setting the distance from the box body to the document panel.