Abstract: The numerical model of carbon fiber reinforced polymer (CFRP) composite sandwich plates with square honeycomb cores of different relative densities subjected to water blast is established based on finite element software ABAQUS. The deformation process, core compression characteristics and structure damage and failure modes were analyzed. The numerical results show that the core compression reaches to its peak value when the velocity of the back face sheet drops to the same as the front face sheet. The maximum core compression increases slowly with the peak pressure when the peak pressure is small and then the increasing rate switches to a higher level until the core is nearly completely compressed. The failure modes of the sandwich structures vary with the core relative density and the value of the peak pressure. The carbon fiber reinforced composite sandwich plates outperform the mass-equal laminate plates when subjected to water blast. The research can provide reference to the application of the composite sandwich structure in the protection shields against underwater shock wave loads.