<p>Blue carbon dots (B-CDs) were prepared via a hydrothermal method using citric acid and urea as precursors, and subsequently employed to construct a carbon dots/MOF-based composite (B-CDs@Cu-MOF) through an in situ one-pot approach. The B-CDs were successfully incorporated into the pores and onto the surfaces of the Cu-MOF, without significantly altering its octahedral crystalline structure, but with a slight decrease in surface area. Due to the reduction nature of the B-CDs, partial reduction of Cu(II) occurred in the synthetic process, resulting in the coexistence of Cu(II) and Cu(I) within the B-CDs@Cu-MOF. The peroxidase (POD)-like catalytic activity of B-CDs@Cu-MOF was investigated systematically using the oxidation of o-phenylenediamine (OPD) in the presence of H₂O₂. The B-CDs@Cu-MOF exhibits distinctly improved POD-like activity, compared to the original Cu-MOF and other previously reported Cu-based POD mimics. The catalytic kinetics parameters including Michaelis constant (<i>K</i>_<i>m</i>) and maximum initial velocity (<i>V</i>_<i>max</i>) were determined. In addition, these B-CDs@Cu-MOF show fluorescence emission at 450&#xa0;nm, attributed to the B-CDs. By integrating it with the fluorescence emission at 564&#xa0;nm from 2,3-diaminophenol (DAP), a product of OPD oxidation, a ratiometric sensing platform was developed. This platform, composed of B-CDs@Cu-MOF, OPD and H₂O₂, was used to detect L-cysteine (L-Cys). In this system, L-Cys competitively consumes reactive oxygen species (ROS) and inhibit DAP formation. Based on that, a ratiometric fluorescence sensor for L-Cys detection was constructed, exhibiting a linear response range from 10 µmol·L^− 1 to 100 µmol·L^− 1, and a detection limit of 1.77 µmol·L^− 1. This work well integrated the enzyme-like catalysis of Cu-MOF and the fluorescence properties of CDs, that provides an excellent ratiometric sensing platform toward target molecules.</p>