Nanomaterial-based second near-infrared region (NIR-II) fluorescent probes have deep tissue penetration. However, it is limited by low catalytic efficiency and specific recognition in the tumor microenvironment (TME). Herein, Ag2S-Cu(II)-Cu2-xSe diagnostic nanoparticles have been established by Cu2-xSe and Ag2S QDs, which can serve as diagnostic agents. They can improve the precise diagnosis of cancer via a NIR-II imaging-guided multimodal photothermal/chemodynamic therapy strategy under near-infrared (808 nm) light irradiation. Under neutral conditions, the fluorescence of Ag2S-Cu(II)-Cu2-xSe nanoplatforms is in the “off” mode, which is selectively turned on only in the tumor microenvironment. Ag2S-Cu(II)-Cu2-xSe NPs not only emit fluorescence specifically in the TME but also have efficient photothermal conversion properties. Significantly, Ag2S-Cu(II)-Cu2-xSe has experimentally demonstrated a strong combined therapeutic effect and can consume excess intracellular glutathione (GSH) through redox reactions. This work presents an effective strategy for achieving precise and safe multifunctional nanodiagnostic platforms.