The utility model relates to parts in a mechanical device, particularly to parts in rolling shutter doors and windows. The utility model discloses a bottom girder for the rolling shutter doors and windows; the bottom girder comprises a shutter piece hook 1, an upper cavity 1, a clamping slot 3, a lower cavity and a bottom slot 5; the upper cavity 2 and the lower cavity 4 are both sealed cavities; the shutter piece hook 1 is fixedly connected on the cavity wall of the upper cavity 2; the clamping slot 3 is located between the upper cavity 2 and the lower cavity 4; and the bottom slot is arranged below the lower cavity 4 and of an open type structure. The bottom girder disclosed by the utility model has a simple structure; through the clamping slot, the bottom girder is clamped into a reinforcing plate or a lock latch, so that a reinforcing effect is realized or the whole rolling shutter doors and windows are difficult to pry, therefore the safety performance is increased.