The aeroelastic stability of the composite wind turbine blade under hydrothermal environment is investigated in this paper. An EulerBernoulli beam model is used to describe slender blade. Hydrothermal effect is introduced through constitutive equation based on composite beam theory. The governing equation of the coupled bendingtorsion (flapfeather) vibration for the composite blade under hydrothermal environment is then established based on the generalized Hamiltonian principle. The characteristic equation of the blade linear free vibration is obtained from the governing equation, and the assumedmodes method is used to calculate vibration characteristics. Furthermore, the effects of twist angle of section, hydrothermal effect and ply angle on blade aeroelastic stability are discussed. The following conclusions are drawn: (1) torsion motion is prone to aeroelastic instability since the damping ratio of torsion is very small, but the aeroelastic stability can be improved by increasing twist angle of section; (2) the increasing of temperature aggravates the instability when aeroelastic instability occurs; (3) the influence of moisture concentration on aeroelastic stability is very little; (4) the effect of ply angle on aeroelastic stability is large.