Practical, high-yield mesoporous organic polymer (MOP) spheres were successfully synthesized by a novel self-template method, which can be used as fillers to prepare poly (ethylene oxide) (PEO)-based polymer electrolytes membranes. The result shows that both the electrical conductivity and lithium-ion conductivity of PEO-based electrolytes can be improved effectively by adding the MOP filler. The electrolyte membranes are mechanically robust, thermally stable to over 270 °C, and can effectively block dendrites penetrating from metallic-lithium anode to cathode. The Li+ transfer conductivity of composite electrolyte membrane increases to 4.4 × 10−4 S cm−1 at 63 °C. All-solid-state LiFePO4/MOP-PEO-LiTFSI/Li cell shows excellent cycling (144 mAh g−1 after 300 cycles) and rate (167 mAh g−1 at 0.2 C, 170 mAh g−1 at 0.5 C, 160 mAh g−1 at 1 C and 145 mAh g−1 at 2 C) performance.