Abstract: The environmental conditions of the wind turbine support structure are complex, and its vibration characteristics and fatigue degree are the key factors to determine the safe operation of wind turbines. The standard method of fatigue calculation is given in the wind turbine design code. Through the long-term monitoring of the environmental data and tower bottom strain of 1.5MW wind turbine, some key problems not covered in the code are studied. First, the relationship between wind speed and tower bottom stress is analyzed. Then, the rain-flow counting method and Palmgren-Miner criterion are used to study the effects of rotating speed on tower bottom stress and fatigue damage under similar wind speed and direction. The results show that the tower bottom stress during turbine shutdown is much smaller than that during rotation, and its fatigue damage is the smallest. The startup and shutdown process will make the tower bottom stress non-stationary, which will strongly affect the stress amplitude and fatigue damage of the structure. In addition, the influence of different selection of representative samples on damage results is also analyzed. Therefore, the load cases should be defined more accurately and reasonable representative samples should be selected in fatigue damage assessment.