Composite polymer electrolytes (CPEs) containing Li7La3Zr2Ta0.5O12 (LLZT) have focused much attention owing to their merits of both ceramic and polymer electrolytes. Nevertheless, due to air exposure, the naturally formed impurity layer on LLZT hinders lithium ion transport, reduces interficial compatibility, and ultimately causes undesirable performance degradation. Herein, a novel and effective method known as in-situ coupled macromolecular bridge is proposed and corresponding functionalized LLZT (LLZT@mPEG) is synthesised. Rigid LLZT cores and flexible ionic conductive polymer side-chains are closely combined by electrostatic interaction, thus resolving the challenge of interface compatibility between different phases. As a consequence, the prepared all-solid-state CPE (LLZT@mPEG-CPE) shows a great ionic conductivity, e.g., 4.9 × 10–4 S cm⁻¹ at 40 ℃ and 7.6 × 10⁻3 S cm⁻¹ at 120 ℃. The Li|Li cell exhibits significant cycling stability of 1750 hours without short-circuits at 120 ℃ and 0.5 mA cm–2. Remarkably, the exceptional thermal endurance is demonstrated by assembled Li|LFP cell with ultrastable performance for more than 500 cycles at extreme temperature of 160 ℃ and high rate of 5 C with a significant capacity retention rate of 94%. This work provides an innovative design principle for advanced all-solid-state electrolytes of Li metal batteries capable of wide-temperature operation.