Abstract: To ensure the normal operation and structural safety of large-scale floating wind turbines under extreme sea conditions, extreme value estimation of structural responses is necessary to be carried out to assess the safety of structures. In this work, the OpenFast software was used to perform numerical simulations of the structural response of a 15 MW large-scale floating wind turbine in the South China Sea under a 50-year return period sea state. Combining with the Poisson approximation assumption, the average crossing rate method was employed to estimate the extreme distribution of tower base shear and bending moment for the large-scale floating wind turbine. The results show that under extreme conditions, the tower base shear force and bending moment will be significantly excited near the natural frequency of the tower’s pitch motion, leading to a sharp increment of the dynamics responses. Moreover, the average upcrossing method can effectively estimate the extreme value distribution of the tower base structural response for a 15 MW floating wind turbine. Under an exceedance probability of 0.01, the estimated extreme values of tower base shear and bending moment based on 1-hour numerical simulation response samples satisfied the structural strength requirements.