The invention provides a loading mechanism for x-ray tube. The loading mechanism for x-ray tube includes an x-ray tube, a swing element and a rotating element. The x-ray tube includes a focal spot location and an x-ray opening. The swing element includes a first rotating axis, and the first rotating axis passes through the focal spot location. The swing element rotates about the first axis to rotate the x-ray tube within a limit swing range. The rotating element is connected to the swing element. The rotating element has a second rotating axis, and the first rotating axis perpendicular to the second rotating axis. The rotating element rotates about the second rotating axis to drive rotate the x-ray tube and the swing element. In addition, the scanning system for three-dimensional image is provided.
