Taking the modification structure design of additionally installed open-type U-shaped desulfurization tower system in a certain 320 000 t VLCC as the research object, combined with the structure drawings of original ship and LR rules, based on the basic principles of structure design, the modification structure design flow and essentials of the desulfurization tower are expounded emphatically, the finite element MPC method is used to analyze and calculate the modification structure strength, and the structure sizes and specifications are optimized according to the results of analysis and calculation.