Most of the pre-bending design of wind turbine blades adopts the static aeroelastic analysis method, ignoring the aeroelastic coupling instability caused by the interaction of blade aerodynamic force, inertial force and elastic force, especially the influence on the flutter performance of overlength flexible blades of 100 m magnitude. In order to compare and analyze the influence of different pre-bending sizes on flutter critical state of blade, a design method of blade aerodynamic/stiffness mapping integrated aeroelastic model based on the stiffness equivalence of the main beam is proposed. The difference between the flutter interval and the critical wind speed of 15MW two pre-bending blades is found by wind tunnel tests. Then, based on BEM-GEBT coupling calculation method, four pre-bending blades are analyzed, and the flutter critical wind speed, aerodynamic force distribution and displacement spectrum characteristics of blades with different pre-bending sizes are compared and analyzed, and the flutter coupling modal mechanism is revealed. The research shows that the results of BEM-GEBT coupling calculation method are in good agreement with those of wind tunnel test. With the increase of pre-bending size, the flutter critical wind speed of flap-edge coupling increases, and the flutter interval range is basically the same. The divergence rates of lift coefficient and pitching moment coefficient of different pre-bending blades are positively correlated with the displacement divergence rate. The average wind pressure curve shows obvious indigenous changes in the pre-bending range of 3m-4m. The flap-edge coupling effect is larger than the flap-torsion coupling effect, and the flutter coupling frequency is dominated by the first-order flapwise frequency.