Abstract: The method for the determination of total dissolved nitrogen in geothermal water by alkaline persulfate digestion-ultraviolet second derivative spectrometry was developed. The total dissolved nitrogen was quantitatively determined in the form of nitrate-nitrogen. The second derivative value corresponding to wavelength 266.6nm was determined as the characteristic absorbance. The parameters in digestion reaction were optimized by single-factor level experiments and a two-factor ANOVA experiment. The digestion temperature, digestion time and solution volume of alkaline potassium persulfate were recommended to be 120℃, 20min and 5.0mL, respectively. The sample volume was 10.0mL and the dilution time was before sample digestion. High content of bromide ions had a positive interference on the determination of total dissolved nitrogen. In the linear range of 0.20-5.0mg/L, the calibration curve of nitrate-nitrogen was established with its coefficient of determination R²=0.9996. The actual samples were determined with the recovery rate (94.0%-103.5%) and the relative deviation (0.83%-3.36%). Compared with the industry standard method, the experimental results were satisfactory. This method has high accuracy and reliability, controllable interference factors, small sample amount, low instrument cost, and can meet the rapid, simple and accurate batch detection of total dissolved nitrogen in geothermal water. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202411210239.