The invention discloses a transportation and installation method and carrier of an ultra-large offshore fan underwater base. A floating dock ship is combined with a floating device to bear the ultra-large fan base which is then transported to an installation position through towboats, so that the intercoastal loading step, the navigation securing step and the marine towing step are finished; and positioning is performed through a dynamic positioning propeller of the floating device, then the fan base is combined with an underwater foundation pile arranged in the installation position in advance, and the preliminary installation step and the positioning installation step can be finished. Accordingly, the installation process is simplified, the operation cost is reduced, the operation efficiency is improved, the international competitiveness of the mainland marine work industry is improved, and the offshore wind power construction is autonomized.