Severe safety concerns and uncontrollable lithium dendrites are major challenges for commercializing high‐voltage lithium metal batteries (LMBs) utilizing state‐of‐the‐art carbonate‐based electrolytes. Herein, a new type of deep eutectic electrolyte (succinonitrile/1,3,5‐trioxane/lithium difluoro(oxalato)borate (DFOB), abbreviated as DEE) with a thermally induced smart shut‐down function is presented to ameliorate the aforementioned issues. In this delicately designed DEE, 1,3,5‐trioxane (TXE) can participate in the Li⁺ primary solvation shell and form an unique solvation structure (Li⁺‐SN‐TXE‐DFOB⁻), which is favorable to enhance the Li/electrolyte interfacial compatibility. It is demonstrated that a 4.45 V LiCoO₂/Li battery using the as‐prepared DEE electrolyte delivers a high Coulombic efficiency (99.5%), remarkable cyclability (capacity retention of 81% after 100 cycles) at 25 °C. Furthermore, thermal abuse tests of LiCoO₂/Li batteries using the as‐prepared DEE display a rapid thermal shutdown function to terminate Li‐ion transportation and subsequent battery operation when they are exposed to environment temperatures exceeding 150 °C. This thermally induced shutdown function is achieved via the polymerization of TXE to polyformaldehydes possessing ultra‐low ionic conductivity at elevated temperatures. The as‐reported smart, thermally‐induced shut‐down DEE opens new avenues for designing next‐generation high‐performance LMBs with enhanced safety properties.