According to enter water the impact problem of air-drop ammunition, the structure vibration theory with relevant solution was used to research on the cylinder vibration response along the axial direction when it entered the water with vertical attitude and received instantly impact loading. The mathematical model was established and dynamic simulation software was applied to simulation calculation the process of water entry and the result were compared and analyzed with the test data, the correctness of the analytical method was verified. The results show that the cylinder material elastic modulus, sectional area, quality and length has a great influence on its response, With the analytic method, the vibration response of the movement body hitting water instant with vertical attitude can be computed, the results can provide theoretical reference to design aerial water entry ammunition shell.