An experiment device was set up to investigate the dynamic responses of water- spacing aluminum plates, and a super-speed camera was used to record the whole process with complex phenomena in different stages. The projectile velocity and the deformation during its motion in water were studied and the failure modes for water-back plate and air-back pates were analysed. It is gained that: the process can be divided into three stages, and the kinetic energy of the projectile transforms to the kinetic energy and potential energy of water; the projectile resistance when moving in water is a constant, about 0.38; the failure modes of the plates is mainly the radial-shrinkage and the successive petal breaking, and the local deformation and petal number of water-back plates are much less than those of air-back plates.