Display omitted •Carbon dots are first synthesized by one-pot method with malonic acid and urea.•The carbon dots possess small size, favorable optical properties, and good stability.•The fluorescence of carbon dots can be selectively quenched by picric acid in 10s.•The strategy improves the sensitivity for detection of picric acid down to 51nM. In this work, novel water-soluble and blue fluorescent carbon dots (C-dots) were synthesized using one-pot hydrothermal method with malonic acid and urea, and were then used as a label-free fluorescent probe for selective, sensitive, and rapid determination of picric acid (PA). The sizes of the as-prepared C-dots were mainly distributed in the range of 1.5–3.0nm with an average diameter of 2.5nm. Moreover, the C-dots displayed blue fluorescence with an emission peak at 395nm (320nm excitation), which also showed good stability and anti-photobleaching ability. Interestingly, the fluorescence of C-dots could be selectively quenched by PA within 10s. Based on this phenomenon, a label-free fluorescence assay was proposed for specific determination of PA. The fluorescence intensity of probe showed a linear response to PA in the concentration range of 0.1 to 26.5μM with a low detection limit of 51nM (S/N=3). Finally, the resultant fluorescent probe was successfully applied to the detection of PA in real water samples.