A radiographic inspection apparatus and a vehicle-mounted security inspection system. The radiographic inspection apparatus includes a scanning device. The scanning device includes: an upright framework; a slip ring rotatably provided on the upright framework; and at least one locking mechanism. Each locking mechanism includes: a driving mechanism provided on the upright framework; and a locking portion provided on the driving mechanism and configured to press the slip ring in an axial direction of the slip ring under a driving of the driving mechanism, so as to prevent the slip ring from rotating relative to the upright framework.