Bisulfite (HSO) is used as a food additive for its antibacterial and antioxidant properties. However, excessive intake of HSO is harmful to humans. Here, for the first time, aldehyde-functionalized dual-emissive carbon dots (D-CDs) are synthesized in one-step for direct ratiometric sensing of HSO. Due to the nucleophilic addition reaction between HSO and aldehyde of D-CDs, the fluorescence transforms from green to deep-blue. The linear range of the probe is 0.1-30 μmol/L with a detection limit of 42 nmol/L. Moreover, D-CDs show good selectivity and a fast reaction time (<5 min) toward HSO. The probe has been applied to trace HSO detection in food samples. The recoveries range from 96.5 % to 107.0 % with relative standard deviations below 6.5 %. In addition, a smartphone sensing platform has been designed, which provides a wider application prospect for the real-time monitoring of HSO in food.