Considering the complex loading environment of the floating offshore wind turbine (FOWT), especially the fatigue damage risks brought by continuous and periodic wave actions, the fatigue performance of FOWT under wave coupling excitation was studied and a long-term fatigue damage assessment method for FOWT was proposed. Taking the Spar FOWT as an example, a nonlinear model under the wave coupling excitation of 8-DOF (dgree of freedom) was established based on the Lagrange equation, and the accuracy of the model was verified. Subsequently, on the basis of the established nonlinear model, the fatigue performance of the example FOWT was discussed according to the proposed method. The results indicate that the fatigue damage of FOWT is closely related to the wave load characteristics, and different damage performance is exhibited under different working conditions. Due to the randomness of sea conditions, only short-term fatigue estimation of the wind turbine is not enough to accurately understand its fatigue performance, and long-term fatigue analysis is needed. Moreover, the peak of short-term fatigue damage at the root of the wind turbine tower occurs near the tower's natural vibration period, while the peak of long-term fatigue damage occurs within the range of high probability sea state periods in the sea. Therefore, efforts should be made to avoid the natural vibration period of the FOWT coinciding with the peak period to prevent high-level damage accumulation and reduce fatigue damage. The analysis also demonstrates the effectiveness of the proposed Monte Carlo based long-term fatigue calculation method for FOWT, which not only has high accuracy but also less time consumption. The proposed improvement method can reduce output fluctuations, enhance stability, and provide more precise results.