The Very Flexible Aircraft (VFA) may result in large elastic deformation during flight, which has a great influence on aeroelastic properties and flight dynamics characteristic of the aircraft, especially on its stability. Based on the threestage rigid hypothesis, the longitudinal dynamic model is developed, where the dihedral angle is employed to describe the wing bending deformation. The relations among the control surface, thrust, angle of attack and dihedral angle are further examined, with emphasis on the influence of dihedral angle on the flight stability. The results show that the dihedral angle has a great effect on the stability for unchanged flight speed and height. Excessive deformation may lead to dynamic instability. In order to maintain the longitudinal stability, it is necessary to control the deformation of the VFA.