Glutathione (GSH) can help the body maintain the function of the normal immune system and its level change is associated with a variety of diseases. To achieve the ultrasensitive assay of GSH, a "switch on" nanosensor is designed on the basis of GSH regulating the inner filter effect (IFE) of MnO2 nanosheets (MnO2 NS) on boron nitride quantum dots (BNQDs). Here, the fluorescence of BNQDs is quenched efficiently in the presence of redoxable MnO2 NS because of the superior light absorption capability; however, the introduction of GSH can trigger the decomposition of MnO2 to Mn2+ and weaken the IFE, causing the partial fluorescence recovery. The recovered fluorescence is dependent on the concentration of GSH. Under the optimal conditions, this sensing platform shows the response to GSH in the range of 0.5-250 μM with the detection limit of 160 nM. On the basis of the GSH activated reduction of MnO2 NS, the MnO2 NS/BNQDs nanoprobes exhibit good selectivity to GSH. The practical application of the proposed system is demonstrated by detecting the GSH in human plasma samples with satisfying results.