By using the shock wave theory, the process that PELE (Penetrator with Enhanced Lateral Efficiency) penetrated a thin metal target was analyzed. The energy loss of the PELE in the process of penetrating the target plate could be divided into the energy obtained by plug when jacket and filling impacted the target, the increased internal energy of jacket and filling, and the shear energy dissipated in the outer and inner edge on the front end of jacket in the process of penetrating the target, etc. Based on the principle of conservation of energy and the method to determine these energies, the approximate formula of the axial residual velocity of PELE was presented. Results show that the axial residual velocity and the calculated results are in good agreement with the experimental data. Compared with the energy loss in different conditions, the filling material has little effect on projectile penetrating ability, and the energy that the plug obtained is the largest while the shear dissipated energy can be neglected.