Abstract: For flexible wing aircraft, the elastic motion of flexible wing and the rigid motion of aircraft were strongly coupled; a dynamic model of flexible wing aircraft was established based on the Lagrange equations. In the characteristic points, the dynamic model was linearized based on the small disturbance linearization processing. Coupled with the time-domain unsteady aerodynamic force model, the longitudinal linear equations of motion in state space form was established. The effects of wing structure stiffness on the longitudinal stability of the aircraft and the dynamic characteristics of the aircraft ware analyzed. The results show that the elastic degree of freedom of flexible wing aircraft would have a great impact on the short period mode. With the increase of flight speed, short period mode frequency will increase accordingly, while the first order bending mode frequency will decrease. When the two frequencies tend to be consistent, the body freedom flutter occurs. The body freedom flutter speed is significantly lower than that of the result based on the cantilever wing model.