This paper aims to perform a time-domain mooring fatigue analysis for a Spar-type floating offshore wind turbine operated in the South China Sea. Tension ranges of mooring lines are achieved from a hydrodynamic analysis where the effects of wind, wave and current are considered. A rainflow counting method is used to calculate the number of mooring tension cycles with corresponding ranges. The fatigue lives of mooring lines are then predicted by Palmgren-Miner’s rule according to T-N & S-N curves. A comparison of fatigue lives predicted by T-N & S-N curves-based approaches with/without considering safety factors is made. The results show that the T-N curves-based approach is more conservative than the S-N curves-based approach if safety factors are not considered in the two approaches, while the fatigue lives predicted by both approaches are in general comparable when the safety factors suggested by API and DNVGL are applied in the two approaches. A comparative study of three kinds of R4 grade studless mooring chains with different diameters (2.5-inch, 4-inch, 5-inch) is also conducted and the results show that the design with the 2.5-inch chain does not meet the fatigue requirements.