Constructing nanozymes with satisfactory catalytic efficiency under physiological conditions is still in great demand for facilitating the advancement of biocatalysts. We herein present a gold nanoparticles-carbon dots nanocomposite (Au-CDs) as an efficient photo-activated nanozyme for monitoring H₂O₂ released from living cells. The integration of CDs with AuNPs remarkably accelerates the catalytic activity at neutral pH via engaging Mn³⁺ ions as the mediators. Meanwhile, the reserved cyclodextrin cavities also enhance the adsorption capacity towards chromogenic substrates through host–guest interactions. Moreover, taking advantage of the inhibitory effect of H₂O₂ on the photo-oxidation ability of the Au-CDs nanocomposite, the Au-CDs based colorimetric method was able to realize in situ assessment of the hydrogen peroxide (H₂O₂) released from living cells. This method paves a new way to establish a promising biosensing platform for unraveling biological events.