As fruitful clean energy, offshore wind turbine power develops rapidly at the coastal area of China that contributes to enabling carbon neutralization. However, the cyclic change of climatic conditions inevitably leads to fatigue issue of wind turbine. This paper makes a survey on the climate condition at Jintang island, Zhoushan islands, China within one year to perform fatigue analysis of in-service composite wind turbine blades. First, the wind velocity rose diagram measured at Jintang island is obtained by investigation, which is used to calculate the wind pressure under some wind velocity and the corresponding direction and frequency, by combining with the modified blade element momentum (BEM) theory. Second, finite element analysis (FEA) of the full-scale composite blade under different wind velocity is performed, where it is almost the first time to introduce the damage model of composites to predict progressive failure properties and stress distributions of composite skin for fatigue analysis. Finally, the fatigue life for blade with three kinds of composite materials for skin is evaluated comparatively by combining with the rainflow counting method, the S–N fatigue curve and the cumulative damage principle. Numerical results show that the fatigue life of blades with three kinds of materials for skin falls within 19–22 years, consistent with the design value of blade in China.