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Diagnostic integration technology represents a significant advancement in cancer diagnosis and treatment. The combination of fluorescence probe-based imaging methods with photodynamic therapy (PDT) offers distinct advantages due to its high sensitivity and minimally invasive nature. However, the effective detection depth and spatial resolution of fluorescence imaging are limited by light scattering effects in biological tissues. Additionally, high concentrations of GSH in the tumor microenvironment (TME) neutralize reactive oxygen species (ROS) generated by PDT, directly inhibiting therapeutic efficacy. Therefore, developing a highly sensitive, high-resolution fluorescent probe to achieve integrated tumor diagnosis and treatment is of great significance. This study developed an activatable diagnostic-therapeutic probe, MB-2O-MB. In MB-2O-MB, methylene blue (MB) served as both the photosensitizer and signal reporting moiety, while a thiols-sensitive disulfide bond was introduced as the linker and response unit. When the probe reacted with GSH in tumor regions, the disulfide bond broke, causing structural dissociation and releasing free MB molecules. Experiments demonstrated that the probe exhibited strong interference resistance, and high sensitivity (LOD = 57.99 nM) for this reaction. Furthermore, the photoacoustic (PA) signal generated upon probe activation compensated for the limited tissue penetration depth of fluorescence imaging, enabling more precise spatial localization of tumor regions. Therapeutically, upon irradiation with 660 nm near-infrared (NIR) laser light, the released MB efficiently generated singlet oxygen, effectively inducing 4T1 tumor cell death. In vivo data further validated the significant tumor suppression effect of MB-2O-MB via PDT. In summary, MB-2O-MB achieves precise tumor localization and complete eradication through the synergistic combination of NIR fluorescence/PA dual-modality imaging and PDT. This strategy simultaneously overcomes the drug resistance bottleneck and imaging limitations of conventional photodynamic therapy, paving a new pathway for constructing highly selective and potent smart diagnostic and therapeutic systems, and significantly advancing precision medicine.