A dry method installing construction method for an offshore large-tonnage prefabricated abutment comprises the steps that vibration sinking construction is carried out through a large-diameter steel cylinder cofferdam to form a dry construction environment; back pressure beams are arranged on rear pouring holes of the prefabricated cofferdam, hydraulic ejecting supporting devices are correspondingly arranged on steel pipe piles in the steel cylinder cofferdam, and the hydraulic ejecting supporting devices can carry out positioning pre-supporting on the prefabricated abutment through hoisting; pouring butt joint of the rear pouring holes and the steel pipe piles is achieved. According to the dry method installing construction method for the offshore large-tonnage prefabricated abutment, the design is reasonable, the construction efficiency is high, the installing construction of the prefabricated abutment under the dry environment is achieved, precise positioning and high-strength foundation pouring connection of the large prefabricated abutment can be finished, and the requirements of projects for the construction strength, safety and efficiency are met.