Low molecular weight residues in chlorinated butyl rubber (CIIR) medical stoppers can leach into polar pharmaceutical solutions, leading to drug degradation and significant safety concerns. While applying a coating to the CIIR surface presents a potential solution, the highly crosslinked structure and low surface polarity make coating adhesion difficult. In this study, oxygen plasma treatment was used to activate and modify the surface of CIIR, enabling the subsequent deposition of a polydimethylsiloxane (PDMS) barrier layer. The influence of plasma power and treatment time on the performance of the PDMS/CIIR composite coating was systematically evaluated. Under optimal conditions (200 W, 10 min), the composite coating achieves a bond strength rating of 1B, which is a marked improvement over the unmodified PDMS/CIIR coating (5B). Furthermore, after modification, the water vapor transmission rate of the composite decreases from 0.0637 g/(m2·24 h) to 0.0321 g/(m2·24 h) (approximately 50% reduction), while the oxygen transmission rate decreases from 42.65 cm3/(m2·24 h·0.1 MPa) to 35.06 cm3/(m2·24 h·0.1 MPa) (approximately 18% reduction), the absorbance decreases from 0.026 to 0.011,demonstrating excellent adhesion and bidirectional barrier performance. This study provides new insights and technical support for designing CIIR-based barrier coatings in high-end pharmaceutical packaging.