Usually, a whole architectural model contains millions of polygons. Visibility preprocessing is necessary for real time walkthroughs in such an environment. When the viewpoint is in the building, only a small part of polygons can be seen by the viewer, the others are all hidden. The display will be speeded up if people just compute this small part of polygons. In this paper, an efficient algorithm of visibility preprocessing based on the "room" structure is proposed. During the real time walkthroughs, the visible polygons instead of those of the whole building are retrieved from the database and are sent to the hideor render pipeline.