Abstract: The influence of backplane conditions on the size and shape of ceramic cone angle formed in alumina ceramic/metal composite armor during the 12.7 mm penetration resistance was studied by means of ballistic penetration test and finite element numerical simulation, and the formation process and mechanism of ceramic cone were analyzed. The results show that the effect of back thickness is particularly obvious on the size of the ceramic cone. When the back thickness increases, the intersection of main cracks is near the target interface, here the tensile shear crack expansion plays a major role. When the thickness ratio of the back to ceramic thickness is less than 1, every 1/6 increase of the thickness ratio can cause 5% size increase of ceramic cone angle. When the interfacial wave impedance difference decreases, that is, when the back plate material wave impedance increases, the stress wave reflected from the interface weakens, thus reducing the damage to ceramics, and the ceramic fracture cone angle also decreases.