An arm-folding mechanism for use in a vehicle-mounted radiation imaging system. The vehicle includes a vehicle body behind a driving cab. The vehicle body includes a generator cabin and a control cabin. The arm-folding mechanism includes a vertical detection arm, a main arm frame, and a hinged mechanism hingedly connecting the vertical detection arm with the main arm frame. The hinged mechanism includes: a first connection arrangement having a free end and a fixed end fixedly connected with or integral with the main arm frame; a second connection arrangement fixedly connected with the vertical detection arm; and an intermediate arrangement connecting the first connection arrangement with the second connection arrangement so that the vertical detection arm can rotate relative to the main arm frame. The bottom of the first connection arrangement is at the same level as or above the bottom of the main arm frame.