Abstract: In order to obtain the capability of jet impacting and initiating underwater covered composite B, the effect of water layer thickness is studied. Based on quasi-steady penetration theory, the velocity expression of copper jet penetrating into aqueous medium is obtained. A penetrate-blast simulation model is established, and the movement, penetration and initiation of jet to different mediums are analyzed in detail. The velocity and diameter of jet before and after penetrating cover board are calculated. The simulation results show that the critical water layer thickness to initiate Composite B by jet is 261 mm, and the initiation threshold value of composite B is determined by Held to be 18.598 mm³·μs^-2. The results may provide useful reference for underwater damage technology of shaped charge jet.