Most wind turbine blade pre-bending designs use the static aeroelastic analysis method. This approach often overlooks the aeroelastic coupling instability caused by the interaction of blade aerodynamic force， inertial force and elastic force. This over? sight is particularly significant when considering flutter performance of ultra-long flexible blades of around 100 meters. To analyze the influence of different pre-bending sizes on flutter critical state of blade， aeroelastic model of the blade was designed based on the stiffness equivalence principle of the main beam. Wind tunnel tests revealed differences between the flutter interval and the criti? cal wind speed of two pre-bending blades of a 15 MW wind turbine. Further analysis was conducted on four pre-bending blades us? ing the corrected Blade Element Momentum Theory-Geometrically Exact Beam Theory （BEM-GEBT） coupling calculation meth? od. This analysis compared and analyzed the flutter critical wind speed， aerodynamic force distribution and displacement spectrum characteristics of blades with different pre-bending sizes， revealing the flutter coupling modal mechanism. The research shows that the results of BEM-GEBT coupling calculation method align well with those of wind tunnel test. As the pre-bending size increases， the flutter critical wind speed of flap-edge coupling increases， and the flutter interval range remains essentially the same. The diver? gence rates of lift coefficient and pitching moment coefficient of different pre-bending blades are positively correlated with the dis? placement divergence rate. The average wind pressure curve shows significant changes in the pre-bending range of 3~4 m. The flap-edge coupling effect is larger than the flap-torsion coupling effect， and the flutter coupling frequency is dominated by the firstorder flapwise frequenc