Abstract: Ammonium is a key dissolved inorganic nitrogen (DIN) in the biogeochemical cycle of nitrogen in the ocean. Depending on the environment, the ¹⁵N natural abundance (δ¹⁵N_air ) of NH₄⁺ can provide insights into N sources and mechanisms of N transformations in terrestrial ecosystems. A variety of methods have been developed for δ¹⁵N_air determination for NH₄⁺ during the past several decades, such as the steam distillation method, ammonia diffusion method, ion exchange method and chemical conversion method. The former three methods are not unsuitable for low NH₄⁺ concentration in seawater samples. In this study, a pretreatment method was developed for determination of nitrogen isotopic composition in ammonium based on the chemical method by hypobromite oxidation combined with sodium azide. To determine the reagent blank source and the effection on the yield rate and the value of δ¹⁵N_air, the appropriate reagent additions, NH₄⁺ concentration were analyzed. There is no interference from any of the nitrogen-containing compounds tested. Higher concentration samples are readily diluted to lower concentration or taken a small volume. When the NH₄⁺ concentration ranged from 5 to 50 μmol/L, the yield rate of ammonium converting to N₂O was greater than 92%. And the calibration curves of ammonium isotopic standards with different abundances showed a good linearity The slope was 0.465 which was close to the theoretical values of 0.5. The standard deviation of δ¹⁵N_air measurements was 0.35‰ (n=5), and the difference between the measured value and the reference value was 0.26‰. This method was used to determination of the nitrogen isotopic composition of dissolved ammonium in seawater, and δ¹⁵N_air (NH₄⁺) value were between 11.46‰ to 22.31‰ . The standard deviation were between 0.20‰ to 0.62‰ (n=5). This method meet the requirements of ammonium nitrogen isotope analysis in seawater.