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Hydrogel electrolytes with anti-freezing properties are crucial for flexible quasi-solid-state supercapacitors operating at low temperatures. However, the electrolyte freezing and sluggish ion migration caused by the cold temperature inevitably damage the flexibility and electrochemical properties of supercapacitors. Herein, we introduce the concentrated electrolyte into a freeze-casted poly(vinyl alcohol) hydrogel film not only reducing the freezing point of the electrolyte (−51.14 °C) in gels for ensuring the flexibility, but also improving the ionic conductivity of the hydrogel electrolyte (5.92 mS cm−1 at −40 °C) at low temperatures. As a proof, an all-in-one supercapacitor, synthesized by the one-step polymerization method, exhibits a good specific capacitance of 278.6 mF cm−2 at −40 °C (accounting for 93.8% of the capacitance at room temperature), high rate performance (50% retention under the 100-fold increase in current densities), and long cycle life (88.9% retention after 8,000 cycles at −40 °C), representing an excellent low-temperature performance. Our results provide a fresh insight into the hydrogel electrolyte design for flexible energy storage devices operating in the wide range of temperature and open up an exciting direction for improving all-in-one supercapacitors. [ABSTRACT FROM AUTHOR]