Thermocatalytic technologies are the most promising approach for converting plastic waste into valuable chemicals. The Ru-based catalysts are the most active catalysts reported, which are highly efficient for C–C bond breaking during thermocatalytic plastic degradation. Still, significant challenges remain in controlling the selectivity of the valuable liquid product. A trade-off relationship between the activity and selectivity is commonly found during thermocatalytic plastic degradation. Herein, we demonstrated that a Ru/γ-Al₂O₃-Ar catalyst, prepared from commercial γ-Al₂O₃ pre-calcined under an Ar atmosphere, achieved 100% conversion over low-density polyethylene (LDPE) hydrogenolysis and an 85.9% selectivity for fuel-range and wax hydrocarbons at 250 °C for 4 h. In contrast, Ru loaded on the synthesized γ-Al₂O₃ showed only a 17.9% selectivity toward fuel range and wax hydrocarbons, with methane being the predominant product. Comprehensive characterizations revealed a strong metal-support interaction (MSI) at the interface between Ru and γ-Al₂O₃-Ar, leading to the abundance of Ruⁿ⁺ species at the Ru-Al₂O₃ interface. Combined experimental and density functional theory (DFT) computational studies reveal that the incorporation of Ruⁿ⁺ effectively suppresses excessive dehydrogenation into methane intermediates. Simultaneously, it promotes the hydrogenation of hydrocarbon intermediates through a synergistic hydrogen spillover effect, driven by high H* coverage, which ultimately boosts catalytic efficiency.