Solid polymer electrolytes (SPEs) are expected to possess high ionic conductivity and conformal interfacial contact with all cell components for all-solid-state lithium-ion batteries. However, the commonly used in situ separator-assisted approach reduces the ionic conductivity because of the use of inert and non-ion-conducting separators. Here, a facile separator-free dual-curing strategy combining UV-curing outside the cell and subsequent thermal-curing inside the cell is reported, in which the second thermal polymerization process provides improved interfacial properties without sacrificing ionic conductivity. The resulting DC-SPEs possess high ionic conductivity (0.3 mS cm−1 at 25 °C), a wide electrochemical stability window (4.64 V vs Li/Li+), and improved interfacial properties. The in situ-formed DC-SPE can effectively suppress the growth of Li dendrites and achieve stable Li symmetric cell cycling performance at high current density (over 700 h at 0.2 mA cm−2 and 0.2 mAh cm−2). The all-solid-state lithium metal batteries (LMBs) with LiFePO4 demonstrate high coulombic efficiency (>99.93%) and ultrastable cycling stability (900 cycles) at 1C rate under 40 °C. The dual-curing strategy provides a brand new in situ processing method to avoid the use of expensive and inert separators, which can be widely applied to the development of all-solid-state LMBs.