Poly(ethylene oxide) (PEO)-based electrolyte is considered to be one of the most promising polymer electrolytes for lithium metal batteries. However, a narrow electrochemical stability window and poor compatibility at electrode-electrolyte interfaces restrict the applications of PEO-based electrolyte. An in situ synthetic double-layer polymer electrolyte (DLPE) with polyacrylonitrile (PAN) layer and PEO layer was designed to achieve a stable interface and application in high-energy-density batteries. In this special design, the hydroxy group of PEO-SPE can form an O-H^---N hydrogen bond with the cyano group in PAN-SPE, which connects the two layers of DLPE at a microscopic chemical level. A special Li⁺ conducting mechanism in DLPE provides a uniform Li⁺ flux and fast Li⁺ conduction, which achieves a stable electrolyte/electrode interface.LiFePO₄/DLPE/Li battery shows superior cycling stability, and the coulombic efficiency remains 99.5% at 0.2 C. Meanwhile, LiNi_0.6Co_0.2Mn_0.2O₂/DLPE/Li battery shows high specific discharge capacity of 176.0 mAh g⁻¹ at 0.1 C between 2.8 V to 4.3 V, and the coulombic efficiency remains 95% after 100 cycles. This in situ synthetic strategy represents a big step forward in addressing the interface issues and boosting the development of high-energy-density lithium-metal batteries.