A facile synthetic method was used to prepare N-CQDs via the hydrothermal treatment of gelatin. The obtained N-CQDs were characterized by FTIR spectroscopy, XPS, TEM and UV-vis spectrometry; the results showed that the N-CQDs have been successfully prepared. Importantly, we designed a “turn off” fluorescent probe for the detection of Cr(VI) ions; the results indicated that when the concentration of N-CQDs was 10 μg mL⁻¹, the pH was 9 and the reaction time was 20 min at room temperature, the fluorescence quenching degree of N-CQDs was relatively large. The results indicated that N-CQDs showed a wide linear range from 0 to 60 μmol L⁻¹, with a low limit of detection of 348.18 μmol L⁻¹. Furthermore, this method and the diphenylcarbazide method were respectively used to detect Cr(VI) ions in leather and the results were compared, which were satisfactory. It can be seen that this method was simple, low cost and had a higher sensitivity and better selectivity.