Abstract: To uncover the interface proximity effect arising from the interaction between shock wave and near-surface hole, defect, as well as impurity in material in practical application, a simplified mechanism study on the influence of downstream planar heavy-light interfaces on the evolution of a shock-accelerated heavy gas cylinder was carried out through numerical simulation. The findings reveal that when the incident shock impinges upon the heavy gas cylinder, it gives rise to diffracted and transmitted wave systems. These wave systems then interact with the downstream planar slow-fast interface, leading to the formation of wave systems that reflect back and forth between the gas cylinder and the downstream planar slow-fast interface. Significantly, these wave systems not only govern the evolution of the gas cylinder interface but also trigger the generation of jets at the downstream planar slow-fast interface. Under diverse interfacial spacing conditions, the type of reflected waves originating from the diffracted wave system outside the gas cylinder varies at the downstream interface. This, in turn, modifies the sequence in which the reflected wave system and the focused wave system inside the gas cylinder interact with the interface of the gas cylinder. When the interfacial distance is narrow, the gas cylinder jet can permeate the gap fluid sandwiched between the gas cylinder and the downstream slow-fast interface. This penetration, in conjunction with the coupling with the jet at the downstream planar slow-fast interface, bolsters the evolution of the gas cylinder jet. As the interfacial distance increases, the jet coupling phenomenon progressively wanes, and the gas cylinder jet succumbs to the inhibitory effect of the vortex pair within the gas cylinder. With a further augmentation in interfacial distance, the gas cylinder jet experiences a promotional impetus due to the stretching effect of the rarefaction wave system reflected by the downstream interface. Quantitatively, the numerical simulation results unambiguously demonstrate that, irrespective of the interfacial spacing variations, the presence of a downstream planar slow-fast interface invariably augments the development of interfacial width, height, as well as circulation deposition.