All-solid-state lithium metal batteries (ASSLBs) are promising for high energy and safety. Halide-based solid-state electrolytes, characterized by high ionic conductivity and a notably wide electrochemical window exceeding 4.3 V, hold significant promise for compatibility with high-energy cathodes. However, oxygen in cathodes exhibits a strong tendency to interact with the central metal cation in halide solid-state electrolyte, forming an unstable cathode-electrolyte interface (CEI) and leading to cathodic degradations. Herein, a pre-oxidation strategy is proposed for Y based halide solid-state electrolytes, leveraging oxygen to pre-establish robust Y─O bonds within the halide electrolyte structure Li₂YCl_2.5Br_1.5O_0.5 (2LO-0.5). The robust Y─O bonds in 2LO-0.5 effectively hinder uncontrolled oxygen interactions with Y³⁺, which would otherwise lead to the formation of oxidizable YOCl. This stabilization promotes the formation of a thin, stable Y₂O₃-based CEI against LiNi_0.83Co_0.11Mn_0.06O₂ (NCM83). Therefore, the ASSLB assembled with 2LO-0.5 and NCM83 demonstrates an initial discharge-specific capacity of 208 mAh g⁻¹ and retained 80.6% of its capacity after 1000 cycles, attributed to stable CEI film derived from pre-oxidized strategy. This work offers new insights for regulating the non-redox reaction between halide solid-state electrolytes and oxide cathodes, promoting the rational design of high-performance halide solid-state electrolytes.