Abstract: When supercavitating projectiles move at a high speed, the periodic impacts (“tail-slap”) with interior surface of the cavity will generally occur. The interactions between the vehicle and the water/cavity interface are sources of structural vibrations, which affect the guidance of the vehicle and undermine the structural reliability. In this paper, the dynamic equation of supercaviting projectile with no constraints is established and solved by finite element method. The dynamic behaviour of supercavitating projectile operating in tail-slap conditions is obtained. And, the effectiveness of periodically placed stiffening rings in reducing the amplitude of the vibrations induced by tail-slap impacts is investigated by considering the difference in number, thickness and spaces-between for the rings. The results show: supercavitating projectile oscillates periodically | larger ring number causes longer impact duration and the penetration distance for the stiffened body is high compared to the plain body | the amplitude of elastic vibrations on projectile nose decreases as the number and the thickness of stiffeners increase | similarly, the period of oscillation decreases with the increment of the stiffener | as for the influence of space-between of stiffeners, the smaller it is, the effect of reducing the oscillation amplitude of mid-part is more obvious | however, the location of the stiffeners almost has no influences on the vibrations of projectile’s nose.