Hydrogen peroxide (H₂O₂), as a signaling molecule, plays a vital role in a wide variety of signaling transduction processes, aging, and diseases. However, the excessive production of H₂O₂ causes various diseases. Herein, we develop a novel method for H₂O₂ detection in live cells via dark-field scattering spectroscopy with gold triangular nanoprisms (AuTNPs) as probes. The corners of AuTNPs would be gradually oxidatively etched by the strong coordination of Br^• which is generated by enzymatic reactions in the presence of horseradish peroxidase (HRP), bromide ion and trace hydrogen peroxide. Benefitting from the morphological change, the single AuTNP based plasmonic nanoprobe shows notable blueshifts and scattering color changes which could be real-time monitored under the dark-field microscopy. The peak position in the scattering spectra of individual AuTNP blueshifts linearly with the increase of H₂O₂ concentration, and exhibits high sensitivity to H₂O₂ in a large range from 2.5 to 100 μM with a low detection limit (LOD) of 0.74 μM. Moreover, the experimental results were supported by the simulated results via the finite-difference time-domain (FDTD) method. The nanoprobes have been further used for intracellular H₂O₂ detection in live cells. Besides, the etching of AuTNP also provides an alternative method to design novel plasmonic logic chips and write-once plasmonic memories.