Abstract: As wind turbine blades reach the 100-meter-scale, the blade size and flexibility keep increasing, which makes the geometric nonlinear effect on the structural dynamic response of the blade more severe. In order to solve the problem of nonlinear aero-elastic calculation of highly flexible blades, an aeroelastic analysis method for highly flexible blades has been established by coupling the Legendre spectral finite element based geometrically exact beam theory and the blade element momentum theory. The calculation result of a curved beam agrees well with the analytical solution, validating the prediction accuracy of the geometrically exact beam theory. By taking the NREL 5 MW and IEA 15 MW wind turbines as examples, the linear and nonlinear dynamic response of a 61.5 m blade and a 117 m blade under both steady-state and turbulent wind conditions are calculated. The results show that, for the 5 MW and 15 MW wind turbines, due to the negligence of the nonlinear effect, the numerical differences of the flapwise tip deflection and the flapwise root moment are increased by 21.35% and 21.23%, respectively, and there is also a 3.2% difference in the frequency of the first edgewise mode. The nonlinear effect of the 100-meter-scale blades has a great impact on the aeroelastic characteristics such as the blade dynamic response and the edgewise mode, thus it should be fully considered in the blade design to ensure the operational safety of wind turbines.