The invention discloses a steel-structure elevating working platform of a building. The working platform comprises height racks arranged in a stacking manner, an elevating stage sleeves one height rack in a sliding way, the left and right ends of the elevating stage are provided with elevators respectively, the upper end of the elevating stage is fixedly connected with a sliding track, a sliding trolley is in sliding connection onto the sliding track, and each height rack comprises four pillars. Groove connecting ends of one height rack can be connected with projection connecting ends of the other height rack via bolts, so that the two height racks are connected stably and firmly; the elevating stage, the elevator, the sliding track and the sliding trolley are arranged, so that the heightracks can be moved, the height of the elevating platform can be changed flexibly according to practical needs of operation staff, and convenience is provided for use by the operation staff; and the height racks are independent from one another and can be mounted/dismounted simply, the size of the elevating platform is reduced, and the elevating platform is convenient to move and store.