Precise monitoring and quantification of H2O2 is highly urgent and of great significance for biomedicine, food safety, environmental monitoring, etc. Herein, we proposed a facile near-infrared (NIR) excited fluorescent probe composed of upconversion nanoparticles (UCNPs) and non-metallic plasmonic WO3-x for ultrasensitive quantitative H2O2 detection. Plasmonic WO3-x with oxygen vacancy-induced LSPR achieved over 680-fold enhancement of upconversion fluorescence at 520 nm, and also acts as the sensitive recognition site for H2O2. H2O2 quenched the LSPR band of plasmonic WO3-x, further significantly influencing adjacent fluorescence signals depending on its concentration. The probe exhibits a good linear response to H2O2 with a low detection limit (10-9 M) and a wide concentration range (0-50 μM), and shows satisfactory application in the determination of H2O2 in blood and milk. This work may provide new ideas for the development of non-invasive fluorescent nanoprobes and plasmon-assisted biochemical detection methods.