Abstract: Fatigue strength of connections between column and pontoon directly influences the reliability of the whole semi-submersible structure. Due to the frequently occurring fatigue failure of existing semi-submersible units, we carry out fatigue analysis based on simplified method for four type connections which have been used in present design. The connections fatigue life is modified by considering the mean stress accounted for the Goodman curve and Gerber curve. Then we compare the results to find out the optimized connection type. At last, the influence of the fatigue life on the reliability of the whole semi-submersible units is analyzed based on the data obtained from the Det Norske Veritas (DNV) rules.