Experiments were carried out to investigate the ballistic performance of fiber reinforced plastic(FRP)-steel plates completely penetrated by hemispherical-nosed projectiles at sub-ordnance velocities greater than their ballistic limits. The FRP-steel plate consists of a front FRP laminate and a steel backing plate. Failure mechanisms and impact energy absorptions of FRP-steel plates were analyzed and compared with FRP laminates and single steel plates. The effects of relative thickness, manufacturing method and fabric type of front composite armors as well as the joining style between front composite armors and steel backing plates on the total perforation resistance of FRP-steel plates were explored. It is found that in the case of FRP-steel plates completely penetrated by hemispherical-nosed projectiles at low velocities, the failure modes of front composite armors are slightly changed while for steel backing plates, the dominate failure modes are greatly changed due to the influence of front composite armors. The relative thickness and fabric type of front composite armors as well as the joining style of FRP-steel plates have large effects whereas the manufacturing method of front composite armors has slight effect on the total perforation resistance of FRP-steel plates.