A facile and effective approach to design and obtain polyacrylonitrile-based electrode materials with shape-controlled and high surface area structure is highly critical and stringent to the development of supercapacitors. To solve this problem, for the first time, the mint leaf-like polyacrylonitrile is prepared via a one-step precipitation polymerization. After pyrolysis and activation process, hierarchical porous carbon nanosheets with a high specific surface area of 3292.3 m(2) g(-1) is successfully fabricated. As the electrode material for supercapacitors, it shows a high specific capacitance of 331 F g(-1) at 0.5 A g(-1) and good rate performance of 220 F g(-1) at 50 A g(-1) in 6 M KOH electrolyte. The assembled all-solid-state asymmetric device present a specific capacitance of 147 F g(-1) coupled with an energy density of 52.27 Wh kg(-1) at a power density of 800.68 W kg(-1) and excellent cyclability with 89.5% capacity retention after 10,000 bending cycles. The synthetic strategy can be easily implemented, without complicated procedure and particular template, and thus opens up a new window towards the simple and highly efficient synthesis of well-designed carbon-based materials for supercapacitor applications.