Abstract: Al/PTFE reactive material is characterized by high energy density. The design of reactive-material composite armor based on its impact-induced reaction energy-release characteristics is expected to provide effective interference against shaped-charge jets. To investigate the influence of rear plate deformation behavior on the anti-jet penetration performance of Al/PTFE sandwich composite armor, shaped-charge jet penetration experiments were conducted under two conditions: the witness target was in contact with the rear plate of the composite armor, and the witness target was spaced 100 mm from the rear plate. A 45# steel sandwich composite armor with the same areal density was also tested for comparison. Meanwhile, a numerical simulation model of shaped-charge jet penetration into Al/PTFE sandwich composite armor was established, and simulations were carried out for witness-target-to-rear-plate spacings ranging from 0 to 100 mm. The results show that, when the rear plate of the Al/PTFE sandwich composite armor is in contact with the witness target, the penetration depth of the residual jet into the witness target increases by 30.2% compared with the condition in which the rear plate can fully deform. In contrast, for the 45# steel sandwich composite armor with the same areal density, the rear plate constraint has no significant effect on the residual penetration depth. The Al/PTFE sandwich composite armor mainly interferes with the jet by driving the deformation of the front plate and rear plate through the reaction energy release of the reactive interlayer, causing repeated interactions between the plates and the jet elements. Compared with the front plate, the rear plate interacts with the jet over a generally higher velocity range and therefore produces a stronger reduction in jet penetration capability. With increasing deformation space of the rear plate, the anti-jet penetration performance of the Al/PTFE sandwich composite armor first improves and then tends to stabilize.