Hydrogels with good flexibility and strong hydrophilicity can be candidates for excellent flexible electrolyte materials. However, the poor structural stability, uncontrollable swelling, and lower potential window of hydrogel electrolytes need to be improved. This work combined quaternized gelatin with cross-linked poly(acrylic acid-co-acrylamide) to form a semiinterpenetrating network and gelatinized in situ in a flexible porous wood skeleton. The flexible wood (FW) skeleton enhances the hydrogel and limits the swelling of the hydrogel. In addition, quaternary ammonium groups and FW act synergistically to provide the composite hydrogel electrolyte with a high ionic conductivity of 5.57 × 10 −2 S cm −1 . The composite hydrogel electrolyte can enable the flexible supercapacitor to operate safely in a potential window of 0−2 V. The optimized supercapacitor has a high specific capacitance of 286.74 F g −1 and provides an outstanding energy density of 39.09 W h kg −1 . The flexible supercapacitor shows a capacitance retention of up to 94.6% after 10,000 charge−discharge cycles, indicating dramatic cycling stability. Simultaneously, a capacitance retention of nearly 90% can be maintained by the flexible supercapacitor after 180°bends for 1000 times. A viable idea for developing high-performance hydrogel electrolytes and flexible supercapacitors is provided in this research.