Mesoporous-assembled MnO2 with large specific surface area was prepared by heating the delaminated MnO2 nanosheet slurry with NH4HSO4 at 175 ℃. When the molar ratio of NH4HSO4 to MnO2 is 2.1, the obtained material had the largest specific surface area of 456 m2 g−1, which gives high specific capacitance of 281 F g−1 at a current density of 0.25 A g−1 and excellent cycling performance with about 1.9% loss after 2000 cycles.