Abstract: Based on Miner fatigue damage theory, after the calculation of the statistical amplitude of mooring tension induced by lowfrequency and wavefrequency wave and the damaged mooring breaking strength, the safety assessment for the fatigue life of a semisubmersible platform’s mooring system in extended service was finished. The research result indicates that secondorder resonant motion is the main ingredient that induces the mooring fatigue damage and the damaged mooring breaking strength has a great impact on mooring fatigue safety life. The assessment method of mooring fatigue life in this paper is applicable in similar assessment for other floating platform mooring in extended service.