In this study, a ratiometric fluorescence-based colorimetric sensor was developed for the portable analysis of antioxidant capacity (AOC) using a smartphone. The sensor used carbon dots (CDs) and o-phenylenediamine (OPD) as fluorescence signal substrate and manganese dioxide nanosheets (MnO₂ NSs) as a fluorescence quencher, a catalyst of the substrate. Without a target, MnO₂ NSs effectively quench the fluorescence of the CDs via inner filter effect and could catalyze the oxidization of nonfluorescent OPD, forming a yellow fluorescent product (ox-OPD). With a target, MnO₂ NSs were destructed into Mn²⁺, resulting in the loss of the fluorescence quencher and catalytic function. The fluorescence of the CDs was recovered, and the fluorescence of the ox-OPD decreased. The fluorescence color change of the system from yellow to blue was further digitized using a smartphone via the red–green–blue (RGB) analysis. Choosing glutathione as the proof-of-concept target, the two readout modes of the fluorescence spectra and RGB analysis achieved the detection limits of 1.84 and 8.33 μM, respectively. Ratiometric fluorescence exhibited precise and reliable detection owing to the built-in self-calibration function, while the colorimetric readout via a smartphone realized the portable determination. This proposed strategy shows a great potential application in assessing the AOC of food samples and controlling food quality.