Due to the high reactivity between the lithium metal and traditional organic liquid electrolyte, the reaction of lithium metal electrode is usually uneven and there are also unexpected side reactions. Therefore, construction of a stable solid electrolyte interface (SEI) is highly essential to improve the performance of lithium metal anode. Herein, a sandwich-like gel polymer electrolyte (GPE) is accurately prepared by in-situ polymerization of Polyacrylonitrile (PAN) nanofiber membrane with trihydroxymethylpropyl trimethylacrylate (TMPTMA) and 1, 6-hexanediol diacrylate (HDDA). The resulting GPE with a tightly cross-linked gel skeleton exhibits high ionic conductivity and electrochemical window of 5.6 V versus Li/Li⁺. In particular, the pretreatment of Li metal anode can improve the interfacial wettability, and the synergy of the chemically pretreated Li metal anode surface and the GPE can electrochemically in situ generate SEI with compositionally stable and fluorine-rich inorganic components. Owing to these unique advantages, the interfacial compatibility between the GPE and lithium metal is greatly improved. Meanwhile, the formed SEI can inhibit the formation of lithium dendrites, and decomposition of GPE would be alleviated. The assembled Li-FEC|GPE|LiFePO₄ full cell shows a high initial discharge capacity of 157.1 mA h g⁻¹, and maintains a capacity retention of 92.3% after 100 cycles at 0.2C.