Research on energy transform rule of armor-piercing on fragment against sandwich plate with fiber reinforced composite cores (is made by superimposing on steel plate、composite material plate and steel plate laminated). The numerical simulation of fragment simulation projectile (FSP) penetrated to different kinds of sandwich plate with high velocity were conducting, the ballistic limits of fragment pierced 16 kinds of sandwich plates were obtained. The credibility of numerical simulation method was verified by comparing with the experimental results. Under fragment critical perforation, the relativity between energy absorption ratio of each part of sandwich plate and structure size was studied by analyzing the numerical simulation results. The research results show that the energy absorption ratio is constant for different thickness cores plate( the aramid fiber cores is 10.41%，the glass fiber cores is 2.68%), the internal energy in core plate is quadratic function increments with the increase of its thickness. Base these, the calculation method of ballistic limit velocity of fragment penetrating sandwich plate with fiber reinforced composite cores was obtained.