A “sandwich-style” composite target with a 45 steel surface plate, polyethylene rubber sandwich plate, and 45 steel back plate is studied for jet penetration resistance. The effects of varying layer thickness are analyzed using AUTODYN software and energy-thickness (E-T) curves in Matlab. The influence of the scale effect on each of these composite layers is analyzed. Results from the E-T curves confirm that varying the thickness of the 45 steel back plate has an obvious influence on jet penetration resistance. Moreover, varying the thickness of the front plate and the sandwich plate has a large effect on jet fracture after penetration of the composite target. Experiments prove that the results of the simulation are correct, and that the scale effect is present for these composite targets under jet penetration.