The monitoring of deferasirox (DEF) has important clinical roles in patients who need iron excretion. However, analytical methods with practicability and simplicity are limited. Moreover, ratiometric fluorescence strategies based on Förster resonance energy transfer (FRET) from carbon dots (CDs) as a donor are rarely reported as a drug monitor. In this work, CDs with an appropriate emitting wavelength at 480 nm and excitation around 370 nm were prepared by hydrothermal approach and HCl post-treatment. O-Phenylenediamine (OPD) can be oxidized by Cu to produce yellow fluorescent 2,3-diaminophenazine (oxOPD) in the system of Cu and OPD (Cu-OPD). Correspondingly, a remarkable FRET from CDs to oxOPD in the system of CDs, Cu and OPD (CDs-Cu-OPD) was fabricated with the quenching illustration of CDs, but emitting property of oxOPD. Attributed to the chelation ability of DEF on Cu, the inhibitory effects of DEF on the Cu-triggered oxidative capability reduced the FRET system by the decreased oxOPD. Thus, the recovered CDs at and decreased oxOPD at were found through a ratiometric mode by the addition of DEF in CDs-Cu-OPD for the DEF assay. The FRET behavior of CDs and oxOPD in CDs-Cu-OPD was proved clearly through the calculation of the association constant, binding constant, number of binding sites, and the distance between the donor and acceptor. Furthermore, this ratiometric method exhibited promising analytical performance for DEF with the application in real samples. The implementation of this work expands the application field of CDs and OPD oxidation in drug monitoring, and even other biological analyses through ratiometric strategy.