The phototheranostics in the second near-infrared window (NIR-II) have proven to be promising for the precise cancer theranostics. However, the non-responsive and “always on” imaging mode lacks the selectivity, leading to the poor diagnosis specificity. Herein, a tumor microenvironment (TME) activated NIR-II phototheranostic nanoplatform (Ag2S-Fe(III)-DBZ Pdots, AFD NPs) is designed based on the principle of Förster resonance energy transfer (FRET). The AFD NPs are fabricated through self-assembly of Ag2S QDs (NIR-II fluorescence probe) and ultra-small semiconductor polymer dots (DBZ Pdots, NIR-II fluorescence quencher) utilizing Fe(III) as coordination nodes. In normal tissues, the AFD NPs maintain in “off” state, due to the FRET between Ag2S QDs and DBZ Pdots. However, the NIR-II fluorescence signal of AFD NPs can be rapidly “turn on” by the overexpressed GSH in tumor tissues, achieving a superior tumor-to-normal tissue (T/NT) signal ratio. Moreover, the released Pdots and reduced Fe(II) ions provide NIR-II photothermal therapy (PTT) and chemodynamic therapy (CDT), respectively. The GSH depletion and NIR-II PTT effect further aggravate CDT mediated oxidative damage toward tumors, achieving the synergistic anti-tumor therapeutic effect. The work provides a promising strategy for the development of TME activated NIR-II phototheranostic nanoprobes.