A simple smartphone-integrated ratiometric fluorescent sensing system for visual determination of copper ions (Cu) and fluoroquinolones (FQs) was developed based on carbon dots (CDs) which were synthesized through the high-temperature pyrolysis of citric acid. In this system, with the fluorescence resonance energy transfer effect between CDs and 2,3-diaminophenazine (oxOPD), the detection of Cu and ciprofloxacin (CIP, an example for FQs) was realized. Cu catalyzes the oxidation of OPD to form oxOPD with yellow fluorescence, resulting in the quenching of CDs. In addition, CIP can inhibit the catalytic activity of Cu and induce the recovery of CDs fluorescence. Under the excitation of 400 nm, the changes of CDs fluorescence at 472 nm and oxOPD fluorescence at 556 nm were monitored. The detection results showed that the sensing system exhibited good selectivity and sensitivity to Cu and CIP with the limit of detection of 2.32 × 10 mol L and 0.2 ng mL, respectively. In addition, a smartphone was developed as a portable analyzer to capture the change of fluorescence color and quickly analyze the concentration of Cu and CIP. The proposed smartphone-based sensing platform has satisfactory sensitivity, and it has application prospects for detecting Cu and FQs in food safety monitoring.