<p>A unified theoretical model that is competent to analyze the stall-induced vibration (SIV) and flutter simultaneously is proposed to investigate the aeroelastic instabilities of wind turbine blades. Hamilton’s principle and the finite element method (FEM) are applied to establish the equation of motion of the blade, and the blade element theory is used to calculate the quasi-steady aerodynamic forces acting on the blade. The aero-damping and aero-stiffness matrices are obtained by linearizing the aerodynamic forces, and the boundaries of the aeroelastic instability of the blade are determined by solving eigenvalue problems. The theoretical model is verified by a wind tunnel experiment, in which both the SIV and classical flutter are measured, and the results show that the inflow angle range of SIV calculated by the theoretical model is reasonable, and the critical wind speed of flutter predicted by this theoretical model is also reasonable and conservative. By analyzing a 5&#xa0;MW wind turbine blade, it is found that the stall-induced flapwise, edgewise and torsional vibrations will occur when the blade is in specific inflow angle ranges. Classical flutter will occur near the 0-degree inflow angle when the wind speed exceeds a certain limit. The vibration frequencies of SIV are close to the natural frequencies, while the flutter frequency obviously deviates from the natural frequencies of the blade.</p>