Oxidative stress in organisms is a factor leading to a series of diseases including tumors and neurological disorders, while methionine sulfoxide reductases (Msrs) may provide an antioxidant and self-repair mechanism through redox cycles of methionine residues in proteins. Thus, it is important to understand the crucial role of Msrs in maintaining the redox homeostasis. However, it remains a great challenge for real-time and quantitative monitoring of Msrs in live systems due to the lack of appropriate sensing tools. Herein, a novel unimolecular platform integrating the intramolecular charge transfer (ICT) and Förster resonance energy transfer (FRET) dual mechanisms was successfully developed. By employing the highly specific Msrs-catalyzed reduction from the electron-withdrawing sulfoxide moiety in the probe to an electron-donating sulfide group, a synergistic ICT-FRET activation process was achieved, leading to a ratiometric fluorescence response toward Msrs with high selectivity, sensitivity, and accuracy. Moreover, benefiting from the favorable features, including mitochondria-targeting, near-infrared two-photon excitation, low cytotoxicity, good stability, and biocompatibility, the probe was successfully used for monitoring mitochondrial Msrs levels in live-neurons, and a positively correlated up-regulation of endogenous Msrs levels under O2•- stimulation was observed for the first time, confirming a Msrs-involved adaptive antioxidant mechanism in neurons. Furthermore, two-photon microscopic imaging of various regions in Alzheimer's disease (AD) mice brains revealed a down-regulated Msrs levels compared with that in normal brains, especially in the cornuammonis of the hippocampus region, which may in turn lead to an aggravation of AD pathogenesis due to the weakened antioxidant and self-repair capability of neurons.