Solid polymer electrolyte is one of the best choices to improve the safety of lithium metal batteries (LMBs). However, its widespread application is hindered because of the low ionic conductivity at room temperature and large interfacial resistance. Here, a cross-linked polymer is synthesized with an unsaturated polyester and used as a polymer electrolyte membrane (PEM). The PEM has a high ionic conductivity (1.99 × 10–3 S cm–1 at 30 °C) and a low glass transition temperature (−54.2 °C), contributing to decreasing interfacial resistance, promoting more uniform Li deposition, and suppressing Li dendrite penetration. The PEM also has a wide electrochemical stable window (∼4.6 V) and superior thermal stability (>150 °C), showing high potential in LMBs. The LiFePO4–Li coin cells and pouch pack batteries with PEM present very stable cycle performance and high safety, indicating that the PEM can be a promising candidate for future solid-state LMBs.