Workers are required to stand on dangerous unfinished steel structures to assemble elements manually. Therefore, we developed a robotic assembly system (RAS) to prevent accidental falls. The RAS consists of four methods: rotation, alignment, bolting, and unloading. The rotation method utilizes a flywheel equipped on top of a rigging beam to rotate the beam. The vertical alignment relies on a camera and a marker to align the altitude of the beam. The horizontal alignment relies on a specially designed shape that can smoothly guide the beam to the right position. The bolting method adds an additional plug hole above each bolt hole to assemble the beam. The unloading method uses pin mechanisms and motors to unload the cable and the RAS. The system is tested in a scaled indoor experiment and the results show that the process is finished without workers stay in the high place. In conclusion, the RAS helps reduce accidental falls, is suitable to the current erection method, and can be broadly introduced to existing sites.