According to IEC61400-3, NREL 5MW Monopile wind turbine and OC3Hywind wind turbine were studied to predict longterm loads of offshore wind turbines. This paper compiles FAST to include the functionality of calculating the wind loads on the tower in the simulation. Two statistics extrapolation methods, i.e. global maximum and block maximum, were used respectively to fit Gumbel distribution to predict the extreme load of out-of-plane bending moment at the blade root, and the results were compared. Further, after discussing different block sizes, the optimal block size was suggested. The comparison of research results with peak-over-threshold method shows that the block maximum method can effectively predict loads, thus providing references for offshore wind turbines load calculation.