Abstract: In this paper, motions of a semi-submersible floating production unit and tension response of polyester taut mooring system are analyzed in dynamic time domain method. The statistical analysis is conducted on the data of platform and mooring system including motion and tension responses. Mooring line fatigue damage is calculated by using Combined Spectrum with Dual Narrow-Banded Correction Factor Method. Drag force and added inertia force of mooring line are calculated based on Morison equation. Sensitivity analysis on drag force coefficient, added inertia force coefficient and polyester EA are conducted to study their influence in mooring line fatigue damage calculation. The results show that the fatigue damage results calculated by dynamic time domain method are more serious than frequency domain method and static time domain; drag force coefficient and EA of mooring line have an appreciable effect on mooring line fatigue damage, while added inertia force coefficient has little effect on mooring line fatigue damage.