Polymer electrolytes have attracted considerable research interest due to their advantages of shape control, excellent safety, and flexibility. However, the limited use of traditional polymer electrolytes in electric double-layer capacitors due to their unsatisfactory ionic conductivities and poor mechanical properties makes them difficult to operate for long periods of time in large-scale energy storage. Therefore, we fabricated a high-performance microporous electrolyte based on poly(arylene ether ketone) (PAEK)/poly(ethylene glycol)-grafted poly(arylene ether ketone) (PAEK-g-PEG) using a certain amount of carboxylated chitosan with a high electrolyte uptake rate of 322 wt% and a high ionic conductivity of 2 × 10⁻² S cm⁻¹ at room temperature. A symmetric solid-state supercapacitor that uses activated carbon as electrodes and a composite microporous polymer film as the electrolyte shows a high specific capacitance of 134.38 F g⁻¹ at a current density of 0.2 A g⁻¹, while liquid electrolytes demonstrate a specific capacitance of 126.92 F g⁻¹. Energy density of the solid-state supercapacitor was 15.82% higher than that of the liquid supercapacitor at a current density of 5 A g⁻¹. In addition, the solid-state supercapacitor exhibited excellent cycling stability of over 5000 charge/discharge cycles at a current density of 1 A g⁻¹. Furthermore, solid-state supercapacitors display lower self-discharge behavior with an open-circuit potential drop of only 36% within 70 000 s, which is significantly better than that of conventional supercapacitors (52% @ 70 000 s), at a charging current density of 1 mA cm⁻². The satisfactory results indicated that the PAEK/PAEK-g-PEG composite microporous polymer film demonstrates high potential as an electrolyte material in practical applications of solid-state and portable energy storage devices.