The invention discloses a building steel calendering mechanism, comprising a platform. A rebar reel, a processing box and a cutting-off box are fixedly connected to the top of the platform sequentially from left to right; a first rotary bar, a second rotary bar and a third rotary bar are movably connected to the back of an inner cavity of the processing box sequentially from left to right. The first rotary bar, the second rotary bar, the third rotary bar, a driven disc, a conveying wheel, a motor, a driving disc, a driving wheel, a first rolling wheel, a driven wheel, an upper derusting wheel,a cross beam, a second rolling wheel, a third rolling wheel, a lower derusting wheel, a hydraulic bar, a cutter and a base are provided herein; therefore, the whole building steel calendering mechanism has high calendering efficiency; the problem is solved that construction progress is affected since service effect of steel material is affected due to the series of problems of existing steel calendering equipment, such as low processing efficiency and unobvious calendering effect.