Abstract: It is known that the air-drop ammunition has to bear instant impact loading when it enters water in horizontal attitude. This paper uses structure vibration theory with relevant solution to calculate the cylinder bending vibration response along the length direction of the ammunition. The mathematical model was established and dynamic simulation software was applied to compute the process of water entry. Results show that the material elastic modulus, section area, quality and length of cylinder have a great influence on its response. The instant bending vibration response of moving body hitting water in horizontal attitude could be computed using proposed analytic method. The obtained results would provide theoretical reference for designing the shell of air-drop ammunition.