Abstract: The trend toward larger and lighter wind turbines has highlighted the risks associated with loads and aeroelasticity. In extreme wind conditions, blade trailing edge windward strategy is one potential load-reduction measure.However, abnormal aeroelastic edgewise vibration remain a concern. To investigate this mechanism, this paper employs aeroelastic numerical simulation and reduction model techniques to study the SDOF flutter aeroelastic instability mechanism of a typical wind turbine airfoil under trailing edge windward conditions. First, through high-precision CFD-CSD coupled simulations, the instability boundaries and response characteristics under different incoming flow angles of attack and reduction frequencies were obtained. It was found that the instability response frequency consistently locks in the structural natural frequency. Subsequently, a reduced-order aerodynamic model for the trailing edge windward conditions was constructed based on the system identification method. Coupling this with the structural motion equations, and utilizing root extraction and root locus techniques, it was discovered that the aeroelastic instability of the trailing edge windward conditions is fundamentally caused by structural modal instability resulting from the coupling of subcritical fluid mode with structure mode. Compared with other SDOF problems, this issue is found to be a phenomenon similar to stall flutter. The study on the influence mechanisms of flow and structural parameters ultimately revealed that the flow mode with low stability margin under the critical wind angle is the primary cause of instability. Adjusting structural stiffness and increasing damping can effectively suppress instability, providing guidance for aerodynamic and structural improvement designs aimed at reducing loads and enhancing stability for large blades.