The invention relates to a steel structure and discloses a steel structure for a concrete building block. The steel structure comprises a first planar frame and a second planar frame which are formed by fixedly connecting prestressed transverse steel bars with prestressed longitudinal steel bars. Connecting pieces are fixedly connected between the first planar frame and the second planar frame. Impellers are rotatably connected between the first planar frame and the second planar frame. An oblique slide rail is arranged between the first planar frame and the second planar frame. A rack is slidably connected into the slide rail. One end of the rack stretches out of the steel structure. The rack is connected with a plurality of gears in an engaged mode. The gears are rotatably connected to the slide rail and connected with pendulum bobs which are located between the first planar frame and the second planar frame. According to the steel structure, compared with the prior art, bubbles generated in the concrete building block pass through the impellers in the pouring process of the concrete building block, the quality of the concrete building block is improved, and meanwhile, the rack drives the pendulum bobs to swing in the pouring process, so that the concrete building block is better in sediment, firmer and higher in quality.