A knob assembly includes a lower housing, a pushing member, an upper housing, and a screw member. The lower housing includes a tank and hook structures. A through via is formed at the center of the lower housing. The upper housing is rotatably stacked on the lower housing. A lower surface of the upper housing includes an annular hook groove. The pushing member is disposed between the upper and the lower housing, and includes a body and pushing structures. A screw hole is formed at the center of the body. When the screw member is screwed into the screw hole through the through via, the pushing member moves toward and locks the lower housing, and the pushing structures push the hook structures to cause the hook structures to expand outward and be hooked to the annular hook groove, so that the upper housing is limited by the lower housing.