The composite polymer solid electrolytes (CPE) were prepared by a solution-casting and hot-pressing method. Na-β-Al2O3 and g-C3N4 particles were incorporated into a polyoxyethylene and polycaprolactone blended polymer electrolyte to obtain the CPE. The microstructures, chemical and electrochemical properties of the composite electrolytes were investigated. The composite electrolyte with a high conductivity at room temperature, a high voltage stability, and a dendrite suppression capability is obtained via optimizing the mass ratio of each component, especially g-C3N4. At 50 ℃, the symmetric cells with metallic Na electrodes exhibit a good long-term cycling stability at a current density of 0.1 mA/cm2. The all-solid-state cells with Na3V2(PO4)3@C positive electrode and Na/C composite negative electrode maintain a specific capacity of about 107 mA· h/g at 0.2 C.