A new dual-mode ratiometric fluorescence and colorimetric probe for selective determination of Cu was developed based on blue-emission sulfur quantum dots (SQDs) and yellow-emission carbon quantum dots (CQDs). The fluorescence and absorbance of CQDs increased in the presence of Cu due to the Cu -oxidized o-phenylenediamine group on the surface of the CQDs. Because of the inner filter effect between SQDs and CQDs-Cu, the fluorescence response of SQDs decreased following the introduction of Cu. Furthermore, in the presence of Cu, the dual-mode SQD-CQD probe showed visible color changes under both ultraviolet light and sunlight. Under optimal conditions, the dual-mode probe was used to quantitatively detect Cu with a linear range of 0.1-5.0 μM for ratiometric fluorescence and colorimetry, with a limit of detection of about 31 nM and 47 nM, respectively. Finally, the dual-mode probe was used for the determination of Cu in practical samples to expand the practical application, and the difference between ratiometric fluorescence and colorimetric methods was compared. The recovery results confirmed the high accuracy of the dual-mode probe, showing that it has immense potential for sensitive and selective detection of Cu in practical samples.