A security inspection apparatus comprising: a radiation emitting device, a back-scatter detector located between the radiation emitting device and an inspected object, a size-distance detecting device located between the radiation emitting device and the inspected object, for detecting a height of the inspected object and a distance from the inspected object, an orientation adjusting device for determining a direction and a height of the radiation emitting device according to the height of the inspected object and the distance between the inspected object and the size-distance detecting device, and adjusting the radiation emitting device according to the direction and the height determined; characterized in that the security inspection apparatus further comprises: a camera, configured to shoot video image of the inspected object and the size-distance detecting device; the size-distance detecting device comprises: judging a height of an obstacle in front of the sensors a calculating device configured to acquire an image height of the inspected object and an image distance between the inspected object and the size-distance detecting device in a video image shot by the camera, and determine the distance between the inspected object and the size-distance detecting device from a ratio of the image distance to the image height and the height of the inspected object detected by the sensors. a radiation emitting device, a back-scatter detector located between the radiation emitting device and an inspected object, a size-distance detecting device located between the radiation emitting device and the inspected object, for detecting a height of the inspected object and a distance from the inspected object, an orientation adjusting device for determining a direction and a height of the radiation emitting device according to the height of the inspected object and the distance between the inspected object and the size-distance detecting device, and adjusting the radiation emitting device according to the direction and the height determined; characterized in that the security inspection apparatus further comprises: a camera, configured to shoot video image of the inspected object and the size-distance detecting device; the size-distance detecting device comprises: judging a height of an obstacle in front of the sensors a calculating device configured to acquire an image height of the inspected object and an image distance between the inspected object and the size-distance detecting device in a video image shot by the camera, and determine the distance between the inspected object and the size-distance detecting device from a ratio of the image distance to the image height and the height of the inspected object detected by the sensors. judging a height of an obstacle in front of the sensors a calculating device configured to acquire an image height of the inspected object and an image distance between the inspected object and the size-distance detecting device in a video image shot by the camera, and determine the distance between the inspected object and the size-distance detecting device from a ratio of the image distance to the image height and the height of the inspected object detected by the sensors.