The characteristics of flap vibration of a wind turbine blade with large deflection were studied. The governing equation of the vibration under unsteady aerodynamic loads, whose expressions were derived from the Greenberg expressions, was established by applying the Hamiltonian principle. The RayleighRitz method was used to calculate frequencies and mode functions. Based on these mode functions, the partial differential equation governing the vibration was discreted by using the Galerkin’s method. The displacement was resolved to static displacement and dynamic displacement, and the equations of the static and dynamic displacement were obtained. The effects of the inflow ration to the static displacement and the aerodynamic damping were discussed. The dynamic response was analyzed by using the nonlineardynamical tools. The results show: 1) that the static displacement rises with the inflow ratio, and it reaches the limit at the blade tip; 2) that the vibration is a harmonic motion for small inflow ratio, and it becomes a quasiharmonic motion when the inflow ratio is big.