The present invention relates to an apparatus for guiding a beam, comprising: a guide box of the beam, through which guide box, an inner space is defined, and having a substantially sector-shaped top and bottom, and an open wide end and an open narrow end; a connecting plate which is connected to the narrow end of the guide box; a first collimator, which is disposed on the guide box in the vicinity of the narrow end and intended to adjust the contour of the beam in the vertical direction and in the horizontal direction; a second collimator, which is disposed on the guide box in the vicinity of the wide end; and an adjusting plate, which connects the connecting plate with a beam generator and is designed to adjust the distance between the beam generator and the guide box. In the present invention, not only the amount and form, but also the centering of the beam are adjustable so that the control quality is improved and the thickness of the shield layer is reduced. The inventive apparatus is applicable to a radiation detection system for security control for liquid items.