Abstract: To simplify the pre-processing process and reduce the detection limit of the method, the study mentioned by the title was conducted. Sediment samples were collected from fish ponds, mixed well, split, freeze-dried, removed from impurities, ground, and sieved. After mixing well, an aliquot (5.00 g) was taken, and 0.2 mL of 10.0 μg·L⁻¹¹³C₆-niclosamide hydrate standard solution and 20 mL of acetonitrile solution containing 2.0% (volume fraction) aqueous ammonia were added. The mixture was vortexed for 1 min, sonicated for 10 min, and centrifuged for 8 min. The supernatant was rotary evaporated to near dryness at 45 ℃, and 2.00 mL of 70% (volume fraction) acetonitrile solution was added to dissolve the residue by vortex. Then, 200 mg of C₁₈ was added, and the mixture was vortexed for 30 s, and centrifuged for 5 min. The supernatant was collected and passed through a 0.22 μm organic filter membrane, and the filtrate was determined by high performance liquid chromatography-tandem mass spectrometry. In chromatographic analysis, the Waters Atiantis^TM dC₁₈ column was used as the stationary phase, and the water-acetonitrile system was used as the mobile phase for gradient elution. In the mass spectrometry analysis, the negative ion (ESI⁻) mode of the electrospray ion source was used for ionization, the selected reaction monitoring (SRM) mode was used for detection, and the internal standard method was used for quantification. It was shown that linear relationship between values of the mass concentrations of niclosamide and peak area ratios of quantitative ions between niclosamide and isotope internal standard was kept in the range of 0.20-50.00 µg·L⁻¹, with detection limit (3S/N) of 0.2 µg·kg⁻¹. Test for recovery was made according to the standard addition method, giving recoveries in the range of 92.1%-113%, and RSDs (n=6) of the determined values ranged from 1.6% to 5.9%. The proposed method was used for the analysis of actual samples, and the mass fraction of niclosamide detected was found in the range of 0.580-2.18 mg·kg⁻¹.