Abstract: Ammonium nitrogen, as an important form of nitrogen, is one of the indicators of water pollution. In view of the complexity of the pretreatment process, low conversion rate, large sample volume and low detection efficiency of the current ammonium nitrogen isotope testing methods, it is difficult to test water samples with low NH4+ concentration. A hypobromite-hydroxylamine hydrochloride conversion method for ammonium nitrogen isotope testing was established and applied to three types of water samples (seawater, river water and soil leachate) for ammonium nitrogen isotope testing. By testing the nitrogen isotopes of ammonium nitrogen standard samples, the conversion rate, precision, accuracy and detection limit of the method were determined and the ammonium nitrogen isotope calibration curve was established. Experimental results showed that the NH⁺₄ concentration range of the ammonium nitrogen standard solution was 2.5~50.0 μmol/L, the conversion efficiency of NH⁺₄ oxidative reduction to N₂O was more than 90% and the standard deviations of δ¹⁵N_(N2O) at different concentrations were within 0.5‰. The slopes of calibration curves of 4 abundance ammonium nitrogen isotope standard samples were 0.489 and the correlation coefficients R² were 0.999, which showed a good correlation. The correlation coefficient R² was 0.999 and the correlation coefficient was 0.999 with good correlation. The nitrogen isotopes were not fractionated during the transformation process. The detection limit of NH⁺₄ concentration in water samples was 2.5 μmol/L. The standard deviations of δ¹⁵N_(⁺4) in the three types of water samples were 0.18‰, 0.27‰, and 0.30‰, respectively (n=5). The differences with that of δ¹⁵N_(⁺4) in the hypobromite-sodium azide transformation were -0.03‰ and -0.03‰, respectively (n=5) and the differences were -0.03‰, -0.03‰ and -0.05‰, respectively, for three types of water samples. The differences were -0.03‰, 0.86‰ and -0.95‰, respectively, indicating that the differences between the test results of the two methods were within the error range and their results could be verified with each other. The replacement of highly toxic and explosive sodium azide reagent with hydroxylamine hydrochloride in this conversion method is more environmental friendly and has higher precision and accuracy, which is able to meet the requirements of isotope analysis and testing of ammonium nitrogen in different types of water samples in the environment.