Flutter is the primary challenge that has to be overcome for the development of super-large wind turbine blade. Although the vibration wind tunnel test based on an aeroelastic model is the most effective solution， similarity ratio and measuring accuracy of the model cannot be solved accurately by traditional methods. In this study， a novel aerodynamic-stiffness mapping integrated three-dimensional complete aeroelastic model design method of super-long flexible blades based on the principle of equivalent stiffness of main beams is proposed for the first time. Later， synchronous full-wind angle wind tunnel tests of vibration and force measurement are carried out using the high-speed photography technology and a high-frequency six-component balance. The nonlinear dynamic response spectral characteristics of NREL-15MW super-long flexible blades are discussed systematically. A comparative analysis of flutter performances and critical instability state of wind turbine blades based on blade tip deflection and blade root reaction force is carried out， which proves the feasibility of forecasting flutter performances according to blade root reaction force. The blade root reaction force method for flutter instability forecasting of super-long flexible blades is put forward. Results show that the proposed aeroelastic model design and experimental method can simulate dynamic performances and flutter behaviors of wind turbine blades accurately and effectively. In the test， it finds that the super-long flexible blades flutter in pitch angle ranges of 93°~96°and 284°~287°. In the flutter ranges， the flutter critical wind speed decreases firstly and then increases with the increase of pitch angles， reaching the valley （5.4 m/s） at 94°. The blade root reaction force and blade tip deflection have consistent divergence and strong correlation. It is suggested that the wind turbine blade enters into the flutter critical state when the flutter index of blade root reaction force is δ≥2%.