Reactive oxygen species (ROS) are key mediators in the progression of atherosclerosis, making ROS imaging valuable for evaluating oxidative stress in plaques. However, most activatable fluorescent probes show limited fluorescence enhancement under physiological conditions because their fluorophores are insufficiently ionized at neutral pH, resulting in weak signal output and low sensitivity. To address this issue, we developed a pKa-engineering strategy by introducing electron-withdrawing fluorine atoms at the ortho-positions of the phenolic hydroxyl group to construct the fluorophore DCM-2F-OH. The difluoro substitution lowered the pKa to 6.0, enabling complete ionization and strong near-infrared fluorescence emission at physiological pH. Incorporation of a boronate ester trigger further afforded the activatable ROS probe DCM-2F-B. DCM-2F-B displayed a 12.8-fold fluorescence turn-on response toward ROS, significantly higher than that of the non-fluorinated analog (2.5-fold), along with a low detection limit (0.03 μM) and low cytotoxicity. The probe successfully visualized endogenous ROS in lipopolysaccharide (LPS)-stimulated macrophages and detected elevated ROS levels in aortic valve tissues from apolipoprotein E knockout (ApoE-/-) mice, consistent with histopathological analysis. This work demonstrates that pKa engineering is an effective strategy for improving activatable fluorescent probes under physiological conditions and provides a useful tool for imaging oxidative stress in cardiovascular diseases.