The invention is applicable to the technical field of bridge construction equipment and provides a mounting operation platform for a cantilever of a steel box girder. The mounting operation platform comprises a moving system, a top frame, a vertical frame, an operation platform and a counterweight assembly. The moving system comprises vertical casters. The top frame is located above the cantilever of the steel box girder, the operation platform is located on the lower portion of the cantilever of the steel box girder, and the vertical frame is located on the side of the cantilever of the steel box girder. The top of the vertical frame is fixedly connected with the top frame, and the bottom of the vertical frame is fixedly connected with the operation platform. The vertical casters are mounted at the bottom of the top frame and abut against the top face of the cantilever of the steel box girder, and the counterweight assembly is disposed on the top frame. The mounting operation platform for the cantilever of the steel box girder is convenient to manufacture and mount and low in labor cost. The whole mounting operation platform can be pushed to move only through labor operation, and accordingly the requirement of construction of the cantilever of the steel box girder of a whole bridge is met.