A hydrogel electrolyte was developed, in which polyacrylamide (PAAm) chains were cross-linked to form network structure, soybean protein (SPI) nanoparticles were coagulated around the polymer chains, and phosphoric acid (H₃PO₄) was incorporated by displacement. The synergistic effect of SPI nanoparticles and PAAm chains makes that the hydrogel electrolyte could maintain structural integrity even after 100 compression cycles at 80% strain; a symmetric all-solid-state supercapacitor was then assembled by using the hydrogel electrolyte as solid electrolyte and two identical polypyrrole-carbon nanotubes papers (PPy-CNTs) as electrode. The assembled supercapacitor possesses excellent electrochemical performance, delivering the maximum energy density of 6.2 W·h/kg and the maximal power density of 398.4 W/kg, which places in the region of standard supercapacitors. More importantly, the device could endure 80% compression strain without fracture or damage, and remain the high capacitance retention, showing a good capacitive stability.