As the third largest power supply mode in China, wind power generation plays a crucial role in the adjustment of the national energy structure and the achievement of the goals of carbon peaking and carbon neutrality. With the rapid development of the wind power industry in recent years, the advantages of the upscaling of wind turbines have become increasingly prominent. However, the flutter problem caused by the long and flexible blades urgently needs to be solved. Based on the finite element calculation under the three-dimensional fluid-structure interaction condition, the dynamic simulation of the flutter process of wind turbine blades was realized, the judgment basis for blade flutter was presented, and thus the critical speed was obtained at which flutter occured. Furthermore, the relationship between the critical flutter speed and the pitch angle was studied, and the dynamic mechanism of stall flutter was analyzed. The results show that the critical flutter wind speed of wind turbine blades shows an increasing trend with the increase of the angle of attack. When the lift frequency of the blade is close to the pitching frequency, the system is prone to flutter. Further stress analysis reveals that when the local stress and deformation phase of the blade are consistent, the aerodynamic force is converted into an exciting force, which triggers flutter and makes the displacement exhibit a divergent characteristic. The research results provide an innovative analytical perspective for the study of the dynamic flutter behavior of large wind turbine blades during operation.