Abstract: The integral main bolt stretcher of the reactor pressure vessel of a nuclear power unit needs to install an elastic cylindrical pin and a bushing on the existing main bolt adapter to realize the functions of screwing in and out of the main bolt. Since the main bolt has been installed on the pressure vessel, using the ordinary knocking method to install the elastic cylindrical pin has the risks of damaging the main bolt hole and the main bolt thread, and the efficiency is low. Therefore, it is necessary to design and manufacture a special installation tool for pressing in the elastic cylindrical pin. Aiming at the on-site working conditions of a nuclear power unit, this paper designs a special installation tool for the elastic cylindrical pin, analyzes the maximum pressing force required for the elastic cylindrical pin, makes theoretical calculations for the key stress-bearing components, and expounds the use method of the special tool. This installation tool was successfully applied in the first overhaul of a nuclear power unit, and completed the installation task safely, stably and efficiently.