The present invention relates to an installation tool for assembling a main rotor shaft unit and an assembly method thereof. The installation tool comprises a first element for supporting the generator end of the rotor shaft and a rotatable support plate is coupled to a second element of the installation tool via engaging coupling elements. The rotor shaft is rotated by a single crane unit via lifting elements mounted to the generator end, and then lifted out of the installation tool and placed on a floor. The bearing units are then mounted to the outer surface of the rotor shaft. The rotor shaft is afterwards lifted onto the installation tool so that the coupling element is brought into engagement and the rotor shaft is rotated on to the first element. This provides an installation tool that eliminates the need for two crane units to rotate the rotor shaft and reduces the amount of man-hours needed to assemble the rotor shaft unit by up to six hours. The installation tool also provides a stable tool for the workers since the center of gravity is located between the first element and the rotation point.