A ratiometric fluorescent sensor for Cysteine (Cys) was synthesized based on Diketopyrrolopyrrole and then characterized by 1H-NMR and MS. The maximum emission wavelength located at 576 nm of the sensor displayed a blue-shift to 536 nm induced by Cys and meanwhile, the fluorescence color changed from yellow to green which could be observed through naked eyes. Moreover, DPP-CHO was experimentally proved a specific sensor to Cys without interferences of other amino acids. Whereafter, its utility was carried out to detect Cys quantitatively according to the liner relationship between concentrations of Cys (0～300 μmol/L) and relevant F536/F576 values of the sensor.