A new crosslinked copolymer electrolyte is prepared with a simply method. The optimal electrolyte presents a maximum conductivity of 1.15 × 10−4 S cm−1 at 60 °C. Especially, this new electrolyte presents an outstanding electrochemical stability not only for a wide electrochemical window (0–5.1 V) but also for a long–term compatibility with the Li metal. Due to electrochemical stability, LiMn0.2Fe0.8PO4 is selected as cathode material cycling between 3 V and 4.25 V. Moreover, a convenient integration method is used in this work to enhance the interfacial contact between cathode and electrolyte. Finally, the all-solid-state battery LiMn0.2Fe0.8PO4/Li shows a maximum capacity of 161.1 mAh g−1 and an excellent capacity retention ratio of 94.5% after 100 cycles. This new crosslinked polymer electrolyte may provide useful insights for introducing high voltage cathode material rather than LiFePO4 only into solid polymer electrolytes system.