Abstract: Equivalent moles of KMnO₄ and Mn(CH₃COO)₂ were used to prepare a weak-crystalline α-MnO₂ supercapacitor electrode material via a mechanochemical route. An assembled symmetrical supercapacitor was tested by galvanostatic charging-discharging within 1.2 V at 200 mA·g^-1. The structure and electrochemical performances of the electrode material were identified by XRD, cyclic voltammetry and AC impedance both before and after charging-discharging. The slope of the charge-discharge curve was analyzed for the first time. It suggests that the supercapacitor exhibits both double-layer capability and pseudocapacitance property. Mn₃O₄, an electrochemical inert, has formed in the charging-discharging process. During cycling, redox peaks disappeared gradually in the cyclic voltammetry diagram, and the pseudocapacitance property disappeared in the discharge curve, then the discharge curve was dosed to the ideal line of double-layer capacitance; the specific capacity and equivalent series resistance of the MnO₂ supercapacitor changed accordingly, the maximum specific capacity of the MnO₂ electrode reached as high as 416 F·g^-l, and retained 220 F·g^-1 after nearly 500 cycles.