Abstract: Based on the analytical method (2d-3DOF method) for two-dimensional three degrees of freedom coupling flutter, the flutter-driving mechanism and flutter modality of the classical coupled bending-torsional flutter phenomenon about thin plate sections were investigated. The research results indicate that the classical coupled flutter is driven by negative aerodynamic damping, and the “aerodynamic stiffness driving mechanism” is incorrect. The negative aerodynamic damping mainly comes from coupling effects between torsional and heaving motions of the whole system. The calculated flutter modality vector at flutter onset reveals that the participation level of heaving motion in the flutter phenomenon is very high, which implies that strong coupling effect exists between motions in different degrees of freedom. Then both the relationship between flutter modality and torsion-bend-frequency ratio and the relationship between flutter modality and structural flutter performance were analyzed. It is found that although there is a simple and unique relationship between flutter modality and torsion-bend-frequency ratio, the relationship between flutter modality and structural flutter performance is rather complicated. The mechanism of heaving-type flutter about thin plate sections was also discussed.