Taken the critical joint structure between the vertical column and deck box of the seventh generation semi-submersible drilling platform as the study case, three design methods for the critical joint structure which can improve the fatigue performance are given. The fatigue strength of three improved critical joint structures is calculated by spectrum fatigue analysis method. The fatigue damage sensitivity of the critical joint structure to different sea conditions and wave directions is analyzed. The influence of different joint structure types and structure welding types on the fatigue life is summarized. The optimal design is provided based on the comparison result of fatigue strength. The optimal design could be applied to the critical joint structure design between the vertical column and the deck box of the seventh generation ultra-deep water semi-submersible drilling platform.