For large-scale offshore wind turbine rotating blades (NREL 5MW), the theoretical model of fluid-structure interaction (FSI) vibration was established, and the blade load and the basic equations for modal analysis were given. Based on ANSYS Workbench platform, the blade modal characteristics at different rotating speeds were analyzed, and further research on dynamic stability was carried out. The results indicate that the FSI and the blade rotation have a great influence on modal frequencies, which increase with the rotating speed of the blade under FSI. When the frequency of the periodic wind speed is close to the first order natural frequency of the blade, the maximum flapping displacement and the maximum Mises stress both increase with time, and the vibration divergence appears. At the safe distance of 4.50m, the critical value of the blade maximum Mises stress basically presents a linear upward trend with the increase of the elasticity modulus, which can provide technical references for optimization design and safe operation of wind turbine blades.