The invention discloses a construction method for prefabricating a composite structure of a pier mounting plate pile under an open marine environment. Layers of walings are arranged; cofferdams formed by steel plate piles are arranged at the peripheries of the walings, and prefabricated piers are hoisted in the cofferdams. Therefore, offshore construction risk can be reduced; the construction efficiency is improved; a dry construction environment is provided for pier installation and node concrete construction; installation of a prefabricated cushion cap is affected by stormy waves due to operation under the open marine environment; after the method is used, the offshore operation time can be effectively prolonged, and all-weather construction can be achieved.