Manganese oxide (MnO2) has been comprehensively studied as one high-voltage electrode material in the neutral aqueous supercapacitors, whereas the working potential window (WPW) for the device hasn't exceeded 2.0 V in most reports. Here, interlaced ultrathin MnO2 nanoflakes were vertically aligned on the carbon cloth (CC) via a facial potentiostatic electrochemical deposition method. A stable WPW for the MnO2/CC electrodes in three electrolytes of 0.5 M A(2)SO(4) (A = Li, Na, K) was defined as 0-1.1 V, which was attributed to the nanoporous morphology of the MnO2 nanosheets and high content of structure water within it. Furthermore, the MnO2 nanoflakes with 0.5 M Na2SO4 have achieved a high specific capacitance of 272.2 F/g at 2 mV/s, and the retention of the performance was 83.8% (5 A/g) after 5000 cycling tests. As for the flexible asymmetric MnO2 supercapacitors (FAMSC), the MnO2/CC electrode was used as a cathode and a self-assembled rGO film was developed as an anode. Working in an extended WPW of 0-2.2 V, the FASMC exhibited a large energy density of 49.8 Wh/kg and the retention of the performance was 88.7% at 5 A/g after 5000 cycles. This work provides a stable WPW for the MnO2 electrodes, which may promote further development of the high-voltage neutral aqueous supercapacitors.