Abstract: Taking the IEA wind 15MW reference wind turbine as the research object,the aeroelastic coupling analysis model of the blade is established based on the modified blade element momentum theory on a polar grid and Timoshenko beam model. And the aeroelastic stability of the ultra-long blades is studied in the runaway situation by combining time-domain and frequency-domain methods. The results show that the blade flutter occurs when the critical rotor speed is 13.06 r/min,and the flutter frequency is 3.68 Hz,whose dominant mode shapes are the third-order forward flapwise mode and the first-order forward torsional mode. Besides,the sensitivities of critical flutter speed to air density,blade mass,section center of gravity,flapwise stiffness,and torsional stiffness are quantitatively analyzed. As a result,it is demonstrated that torsional stiffness is the dominant factor affecting the critical flutter speed,and the flutter margin can be improved by reducing the blade mass,shifting the cross-sectional center of gravity forward,and increasing the flapwise and torsional stiffness. In addition,it can obtain a more conservative design-rated rotor speed by considering high air density.