A high-yield and facile approach for producing K-birnessite layered nanoflake was proposed via simply mixing and hydrothermally heating the mixture of potassium oxalate and potassium permanganate aqueous solution. The product yield was proved to be significantly increased with the addition of potassium oxalate, and the possible reason was also discussed. The product was confirmed to be K-birnessite by a series of characterizations, and fabricated on nickel foam to study the electrochemical behaviors as active electrode material for supercapacitor. Electrochemical results showed that the product exhibited well-performed charge storage properties, and could be promising for applications in energy storage devices.