The back scatter detector is one truncated rectangular pyramid structure comprising one bottom plane, one top plane and four side planes to form one sealed casing. The bottom plane as the X-ray incident window has outer layer of aluminum-plastic board and inner layer of barium fluorochloride screen; and the top plane and the four side planes have transparent flash cesium iodide crystal sheets adhered to the inner surface and mounted photomultipliers. The present invention has barium fluorochloride layer to absorb low energy X-rays and transparent cesium iodide crystal sheets to absorb high energy X-rays, and this can greatly reduce afterglow, raise the X-ray absorbing efficiency and raise light converting efficiency.