In this paper, we develop a facile UV-derived in-situ dual-reaction to prepare a flexible poly(ethylene oxide) (PEO)-based solid polymer electrolyte (SPE), which is applied for ambient-temperature all-solid-state lithium battery. By modifying the amorphous domain of PEO through cross-linking with tetraglyme (TEGDME) and introducing a rigid linear oligomer of tetraethylene glycol dimethacrylate (TEGDMA) into the matrix, a SPE is obtained with high ionic conductivity (2.7 × 10−4 S cm−1 at 24 °C) and enhanced mechanical strength. The as-prepared SPE shows superior electrochemical properties with decent lithium transference number of 0.56, wide electrochemical stability window above 5 V vs. Li+/Li and low interfacial resistance. By means of galvanostatic cycling studies in Li//Li symmetrical and LiFePO4//Li cells, we further demonstrate that the SPE exhibits excellent cycling performance with minimization of lithium dendrite formation.