In this study, we systematically investigate the mechanism of single-layer MnO2 nanosheets suppressing fluorescence of 7-hydroxycoumarin and, based on this, demonstrate a new fluorescent method for in vivo sensing of ascorbic acid (AA) in rat brain. The mechanism for the fluorescence suppression is attributed to a combination of inner filter effect (IFE) and static quenching effect (SQE), which is different from those reported for the traditional two-dimensional nanosheets, and Förster resonant energy transfer (FRET) mechanism reported for MnO2 nanosheets. The combination of IFE and SQE leads to an exponential decay in fluorescence intensity of 7-hydroxycoumarin with increasing concentration of MnO2 nanosheets in solution. Such a property allows optimization of the concentration of MnO2 nanosheets in such a way that the addition of reductive analyte (e.g., AA) will to the greatest extent restore the MnO2 nanosheets-suppressed fluorescence of 7-hydroxycoumarin through the redox reaction between AA and MnO2 nanosheets. On the basis of this feature, we demonstrate a fluorescent method for in vivo sensing of AA in the cerebral systems with an improved sensitivity. Compared with the turn-on fluorescent method through first decreasing the fluorescence to the lowest level by adding concentrated MnO2 nanosheets, the method demonstrated here possesses a higher sensitivity, lower limit of detection, and wider linear range. Upon the use of ascorbate oxidase to achieve the selectivity for AA, the turn-on fluorescence method demonstrated here can be used for in vivo sensing of AA in a simple but reliable way.