The utility model discloses a prevent half space that falls, including uncovered shape joist frame and income oral area, uncovered shape joist frame sets up the diapire at the income oral area, the outer one side of joist frame sets up switching structure, its characterized in that: one side bottom that the switching structure was kept away from to the joist frame is provided with the joist extension, the diapire on one side with the joist extension is connected. The utility model provides an unable problem that covers of entry that exists of current half space, through arranging the platform that needs the staff to stop on the platform joist, the platform have satisfy distance that the operation personnel pass through and can be when the rest of operation personnel effective closure, guarantee operation personnel's personal safety.