Activated carbon (AC) was prepared from lignite by microwave heating ZnCl2. The pore structure parameters of AC are characterized by nitrogen adsorption technique. The AC and ruthenium oxide/AC composite are characterized by thermogravimetric analysis and transmission electron microscope. Electrochemical properties of ACs and ruthenium oxide/AC composite electrodes were investigated by cyclic voltammetry and constant current charge–discharge after AC was pre−oxidized by HNO3 solution. The results show that the specific surface area and total pore volume of AC from lignite reaches 1310 m2 g−1 and 0.80 cm3 g−1, respectively. The micropore volume of AC from lignite totals only 12.5%. AC and ruthenium oxide/AC composite electrodes with 5wt.% ruthenium oxide loading show high cycle stability. Compared to pristine AC electrode, specific capacitance of ruthenium/AC composite electrode and energy density of ruthenium/AC capacitor after 100 charge−discharge cycles increases 40.8% and 39.1%, respectively.