The invention discloses a method in the technical field of shipping hoisting, in particular to a modification method of a large-scale crane ship based on a large-scale
oil tanker with longer parallel middle body. The floating force is increased through assembling a modified ship bow and box-shaped floating bodies at the two sides of a ship body, and the modification of the large-scale crane ship is realized through replacing a main
deck by a beamless flat
deck, arranging a crane and reinforcing the interior of the ship body. The invention meets the requirement on controlling the floating state during the working of the large-scale crane ship, improves the total
longitudinal strength of the shipping through the arrangement of a modified
deck and a centerline bulkhead, and meets the requirement on local intensity through the design of a
barrel-shaped
honeycomb structure. Rapid dispensation of
ballast water during working is realized through the optimized design of a
ballast tank and the arrangement of a
ballast water pump; the
work performance and the control performance are improved through configuring an auxiliary
machine, an anchoring
machine, a
winch and a bow thruster so as to realize that the large-scale
oil tanker is modified into the large-scale crane ship on the basis of little modification, therefore, the time and the cost are saved; and the method has larger economic effect and finds a new way for developing old ships.