Na+-intercalated MnO2 is synthesized via hydrothermal reduction. For comparison, the pristine and K+-intercalated MnO2 are also synthesized and characterized. Cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy methods have been carried out to study the electrochemical properties of the obtained MnO2 electrode. Na+-intercalated MnO2 electrode shows the improved Na+ diffusion coefficient and electrical conductivity, which is suitable for supercapacitor. A specific capacitance (based on Na+-intercalated MnO2) of 295 F·g−1 at a current density of 1 A·g−1 is obtained in Na2SO4 aqueous solution. Such results suggest that pre-intercalation of alkaline cations into Mn oxides is an effective method to enhance the electrochemical activity in alkaline electrolytes. Moreover, an all-solid-state symmetric supercapacitor based on Na+-intercalated MnO2 electrode is assembled and tested.