With the Chinese medicinal material Radix Scrophulariae as a carbon source, the fluorescent carbon quantum dots are prepared by the high temperature calcination method. The carbon quantum dots solution under the excitation at 270 nm wavelength has the maximal fluorescent light emission wavelength at 418 nm. The fluorescent light intensity of the carbon quantum dots solution can be effectively quenched by the introduction of the nitrate ions (N O3-). Based on this, a novel method for detecting N O3- is developed using carbon quantum dots as the fluorescent probes. Under the optimal experimental conditions, the signal shows a good linear relationship with the concentration of N O3- when this concentration is in the range from 0.4 μmol/L to 80 μmol/L. The limit of detection is 65 nmol/L. When the 5.0 μmol/L or 50.0 μmol/L N O3- solution is added to the real water samples for the standard adding recovery experiment, the recovery rate is in the range from 98.0% to 104.0%.